My cat seems to have a dermatological issue, all vets I’ve seen can’t seem to find a solution

Hello,

My cat is 10 years old and for the last 2 years he has had this issue where he will over clean his skin until the point it bleeds and there’s no fur in the area anymore, the places are all random, some on his back, legs, paws, face. It seems like that areas must be itchy as he really goes for it. He spent the entire of last year wearing a collar at his vets recommendation but it didn’t help as he always found ways around it and still managed to break the skin. I’ve spoken to 3 different vets who haven’t been able to diagnose but have said the collar will fix it. Is there anyone here who can tell me if they have seen anything like it before? We still have him in a collar for the majority of the day but it’s obviously not working and I hate to see him like this.