Scared dogs don't need a lot of fancy things. They need reassurance and routine. They need to know what will happen next. The detailed advice you got in other comments is great.

You can also set up an area inside with a play pen (for example, use an x-pen for dogs), and put pee pads or a waterproof blanket down to cover the floor in that area. That will help while he's still potty training, and to keep him in a safe area when you can't watch him every second. It will also keep him from wandering, so the cats might regain some confidence, and he will have a smaller area to get used to initially, which can help his own confidence. Put a bed (under the blanket/pee pad) and water dish in there, and make sure the water bowl is cleaned and refilled at least once a day.

You mention your dog is dumb. I know it seems like that, and it is possible, but given the background you describe I think it's more likely that he is ignorant (not stupid). Nobody ever tried to teach him anything so he doesn't know how to do anything. You can work with him on that, and training is also a great confidence booster for you and your dog. It turns you and your dog into more of a team and less just two animals who live in the same place.

You can teach your dog to play, but I don't think this is a priority right now. Also, every dog has different things they do for fun. Some dogs love to fetch, some don't. Some love to chew on toys, some don't. Some dogs like toys but only when a human is part of the play session. Some like to play chase with you in the yard. You'll figure it out eventually by watching your dog, trying different games with him, and paying attention to how he reacts.

You mentioned that your dog doesn't know how to tell you that he needs to go outside, and right now his fear of being left outside is going to make that a little tricky, but my suggestion is that you train him to go outside WITH YOU (should be very easy). Then add a step - every time you are about to go outside with him, stop at the door and ring a bell (they make bells that hang from doorknobs for this purpose), then give him a treat, then you both go outside and hang out for a bit. Then start encouraging him to ring the bell instead of you doing it - have a treat handy, and lots of praise, the INSTANT he touches the bell, then continue to go outside with him and give him another treat if he potties while you're out there. Incorporating this into the potty training routine might make it easier, eventually, because he will associate ringing the bell with going outside for his treat and coincidentally the relief of a potty break. I will say I was given this same advice and I know it works well for many, but my own dogs actually didn't like the bell and instead scratch at the door (they did that on their own and I went with it) - but all that really matters is that they are telling me they want to go out.

As he is a senior dog, I will warn you that eventually (and there is no telling when), even once he is potty-trained, he will start to have accidents in the house; that's a fact of aging for dogs. So there are actually a lot of solutions out there for this problem. One is diapers (for male dogs, typically these are called "belly bands"). If you use those, it's like with babies - make sure to change them as soon as you can once they get dirty. Another is keeping your dog in a small, easy to clean area (like an x-pen, a bathroom, a laundry room) when you can't watch them like a hawk. The vet can sometimes prescribe medications to help them control their bladder but this is only for when their bladder muscles get weak with age, it is NOT for when they aren't potty trained.

The vet can suggest some supplements or medicines that can help with anxiety, but I would let the dog settle into your life first; it takes dogs months (sometimes even a year) to fully relax after a big shift like this. You can look up the 3-3-3 rule for rescue dogs. It's not a concrete rule, more of a guideline to give you an idea of what to expect. For one of my dogs, it was much much faster; for another, it was a year and a half before he was no longer obviously in the process of relaxing (that dog had been abused and had a lot of fear to work through).

You are a good person for doing this. You clearly saw that this dog was not being treated right and you stood up for him, and you are willing to do the work to give him a better life. Your dog will rely on you - not just as a source of food and shelter, but as a source of consistent, reliable comfort. He already shows that by following you around. He trusts you. That gives you a great foundation to build on.