If its not fleas, tick or anything else that could be irritating him. I think it might be a behavioural issue. My cat went through a weird phase of spending hours licking his belly it got to the point where he was completely bald on his belly. We though he had fleas or an irritation took him to the vet and he just said that it seems that hes obsessed with cleaning himself. Because he would spend hours licking he would have horrible fur balls and would throw up his food. He started to lose weight. We rubbed some stuff on his belly to stop him licking and he ended up licking above his tail bald. That was about 4 years ago and now he seems to be alright now. He sometimes has his weird days and we kinda knock him so he stops doing it.

I hope you figure out whats wrong with your kitty