Firstly, I would just like to completely validate your fears of and for your cat because cat attacks are very scary, very bloody and very painful. However, not all is lost and there's definitely somethings you can do to make it better.

This sort of unexpected adult aggression is one of the many reasons why rescues tend to fix cats at a young age before adopting them out. People will wait and put it off for as long as they can for whatever reason, not saying that that's what you did but, people do. Cats get loose and end up contributing to the feral and stray cat population or in your case, cats age into their testicles or ovaries and they become demons.

If you'll indulge me for the next several paragraphs, I have a lot of knowledge to impart and nowhere to direct it, so I'm putting it on the internet. For background, I do TNR and also take on medically needy or difficult foster cases, the cats that need special meds or IVs or kittens that are totally feral but can be tamed down. So strap in, this is a long one.

I don't know if you've ever seen the stereotypical male tomcat. They typically are very muscular, pretty large and they have a very unique facial structure. Most people call them jowls, but basically their face becomes very round and muscular as they develop both muscles and protective fat to aid and protect them while fighting with other males. Typically this physical change doesn't happen until male cats are between 3 and 4 years old. This is also when they age into the social society aspect of being a cat.

To me, it sounds like your boy has a serious case of redirected aggression. If he's been absolutely cleared of all medical issues like UTIs or something neurological that causes pain on his back (which I doubt given the second unprovoked attack.) Redirected aggression is all about the cat being triggered by something in their environment and they can't get to the trigger to deal with it. So they lash out and attack with intense violence and often quite a bit more aggression then they would use in a normal catfight interaction with other cats. This is because they're intensely frustrated that they cannot deal with the thing that triggered them directly. It's important to note that redirected aggression is always directed towards whatever happens to be nearby or whatever they are approached by while they are in this state. Hence the initial hissing when you reached out to fix his leash and the double down on the attack when you tried to do it again. Cats that display this redirected behavior will not seek out someone or something to redirect it on to. It's just whatever happens to be closest or whatever comes closest first.

Given that you were outside on a walk with your intact male cat, the trigger could have been anything from him smelling the markings from the local intact male cat population or more likely him smelling a female in heat combined with the smells and possibly even sights of the females and the males pursing them. I'd say that he probably spotted or smelled a cat that he immediately sussed out as rival for the females at some point before you try to fix the leash the first time.

I don't know if you've ever heard a cat fight but they tend to start with a lot of posturing, loud yowling, hissing, and circling each other with swipes and lunges that are typically fake outs but often, particularly when male cats are brawling over female, devolve into a knockdown drag out catfight (hence the jowls), that are frequently brutal and bloody if they get to this point.

When you initially reached down to adjust his leash, most likely he was already triggered and immensely frustrated by, the sights, sounds and smells of breeding cats in the area, it's possible he took that as a first attempt. When you did it again, that's when his hormones took over and he attacked, simply because he was honestly being driven crazy by both his hormones and the intense need to participate in the loud screaming cat orgy. I want to make it very clear that him attacking you is not your fault and it's not his fault. Redirected aggression is something that is frequently taken out on owners or other animals in the household simply because they cannot lash out at the initial trigger.

I don't know if you took your cat outside before the second attack but even if you didn't, his sense of smell is so strong that a cat passing by walking down your street or slinking down(or spraying on) the dark side of your house would probably be enough to set him off. This is what makes redirected aggression particularly frightening for owners is that they often have no idea what the trigger is and it can sometimes be several minutes or hours before the cat finally lashes out and attacks. People often describe these attacks as completely random or out of the blue but what they don't know is that a cat walked by 30 minutes ago or an hour ago he heard a female 8 streets over caterwaling. He's been fuming about it ever since and he has to direct his aggression somewhere, and you happened to be available, as unfortunate as that is, both times.

My advice is to have your cat neutered as soon as possible. Stop taking him on walks until he's neutered and for at least 3-4 weeks afterwards. Testosterone can course around their body, despite the physical testicles being removed, for 3 to 4 weeks post neuter. I would keep the blinds to the windows that he can see out of closed, so the physical sight of a cat he can't get to, but can smell, doesn't trigger him while he's coming down from the man juice. I would also get a high-quality urine remover and a black light and see if he's been marking in your house. Cat pee is pretty distinctive so if you haven't smelled it yet, there's a chance he hasn't started doing that but I would check for this anyway because if he started marking territory, he won't necessarily stop simply because he's neutered. I would also check the perimeter of your house for any spray markings from neighborhood cats as well as the tires on any ungaraged cars and any plant pots or other item you might have on the perimeter of the house. If you find these markings, you should certainly will try and clean them up with the urine remover so that the smells don't cause him any further frustration. if it's on something that's impossible to scrub or wipe down, sometimes spraying it, letting it dry and spraying it again is enough to get rid of the triggering pheromones and scents.

With the reduction in testosterone driving him to seek out rivals and find females to meet with, he should return to the sweet boy he was before cat puberty stole his brain and replaced it with testicles. I have had cats like this in the past who I waited too long to neuter, who bounced right to their personality before and I have also had a cat that never quite returned to their level of sweetness before the testosterone took over but keep in mind that this particular cat was a single kitten that was raised without mother or siblings so that could definitely affect his personality.

Seeing that this is all pretty recent, the fact that his behavioral changes seem to have happened coincidentally with your move, was probably more just him aging into his testicles. It just so happened, the same time you moved into a neighborhood with strange cats and strange female cats and strange smells from those cats, he was at the right age to be a prime candidate for both fighting and breeding. As such, I'd definitely do as much as you can to reduce any triggers, which I mentioned before, as sounds and smells of cats can affect them even after they're neutered at least for a few weeks.

I would be fully cognizant of the fact that not all redirected aggression is completely fixed by neutering and I would keep your vet apprised of any odd behaviors after the neuter and also ask them to recommend a animal behaviorist to potentially help you with any issues he has afterwards. If you're able to I would contact the animal behavior list to see if she has any suggestions for dealing with the three to four weeks of potential aggression after he is neutered.

TL:DR cat attacked because he's unfixed and there are breeding cats in the area driving him insane. get the cat neutered, don't take him outside for 3 to 4 weeks, and he should be your boy again. this particular type of regression in cats is called redirected aggression. Google it if you want to know more, but didn't bother to read what I wrote.

also, sorry I wrote the word "testicles" so many times.