why is my cat bald on his neck??

first and foremost, his collar is NOT too tight. I can easily fit 3 fingers underneath it. we thought it was the flea collars and that he maybe had an allergy, so we switched to regular collars. it's been months, and it still hasn't grown back. he doesn't have fleas or ticks. the vets don't know why this is happening. he was going bald on some other parts of his body about a year ago, and we put it down to stress since the neighbour's cats don't leave him alone. he was given some kind of steroid injection at the vet's, and all the hair quickly grew back, except under his collar. he's been bald on his neck for absolutely years now. longer than the neighbour with all the cats has even lived here. whatever it is, it's not the same cause as the hair loss on the rest of his body last year. we know he is disabled and a runt (he has 3 and a half paws, asthma, and is abnormally small) so it might be something to do with that in terms of maybe skin sensitivities because of dodgy genetics?? he also went a while without any collar at all (not ages, but long enough that his hair should've at least grown back a bit) and nothing happened. why is he bald?