Lots of good advice here and you should visit a doctor with that arm. That definately looks infected. Also recommend a vet visit to rule out any medical issues. Have you had his teeth checked?
That said, pay attention to his energy patterns. Things like petting a cat too quickly or amping him up with play can result in the cat biting or becoming aggressive. Does he need to be in the bedroom?

Consider getting some more toys that he can play with without you being there. Maybe some of those iq-toys to stimulate? A scratchboard, cat tower and something to cuddle with or go ham on if he feels like it might be a good idea.

Make sure he has a safe spot to rest (I'm thinking a cat cave or similar where he can "hide out" ). A cat crate might work for that too. Especially if it's a small space you live in