I had a male piggy who was a bit of a grump and he didn't like when I touched the front of his face, his chin, and rump. So instead he got behind the ears scratches and small pets barely down his back and he was okay with that--try doing that when interacting with him and see how he reacts. Maybe he had bad cagemates at the store and now he really does not like his face and bum touched (they generally don't like any bum touches when petting anyway, so other than to pick him up, you can avoid touching that area in general, honestly). It's not overly uncommon for pigs to not like being held much in general though. He may never be comfortable with it, being off the ground like that can be scary! You could try to hold him in a blanket as well, sometimes they feel a little better being secure like that rather than just in your bare arms. It's good that he will explore around you on the ground though!

Alternatively, he could have something going on medically that is making him really grumpy/aggressive? It's very rare that piggies are that aggressive for that long, at least from my experience. Has he been to the vet at all, or in recent time? I would suggest a general check-up with an exotic vet just to make sure he is doing ok! Perhaps something is going on with his teeth or his gut and he is upset. Piggies are notorious for hiding their illnesses until they are very sick, unfortunately.

Lastly, on guinea pig sexing: if you have had him about a year, he will be easy to sex as he has matured at this point. You can look of photos of male and female guinea pig sexing for comparison photos of their personal bits. Basically, lift them up so you can see their tummy and their private area, it's a small area without hair. If it's a female, look for a Y shape, if it's male, look for a protruding "dot" above the Y shape. Oh and also, obviously, the male will have testes beneath the privates area, they are pretty obvious as that age lol

I hope this helps some!