My cat keeps scratching hive neck to the point where there are patches of fur missing and he bleeds

For background: We know he is allergic to something in chicken so we completely avoid that. He eats a limited ingredient diet, fish only. We give him wet food as well.

He is a big boy, at 20lbs. Though he doesn’t eat muc, his brother eats 2-3x as much and is half the weight.

The vet told us that for his breed his weight is ok though it wouldn’t hurt to lose a bit. (We did every test under the sun to find out why he gained so much)He is very active, playful, running around all the time and jumping all over the place. He gets time outside and he is fully vaccinated for everything.

The vet said that the scratches are definitely self inflicted but unsure why he’d do that. We would try a cone but we’re afraid that it will irritate his neck more or he will injure himself even more trying to get it off

Any advice or help or insight would be appreciated!!

Thank you!