If he still wears the same collar and it is the culprit, it will irritate already raw skin and prevent complete healing. I know the holidays are usually tight for everyone, but perhaps finding a (maybe no-so-stylish) collar made of a completely different material and without dyes would stop the cycle. That's what eventually worked for Elvira, my cat. If it is confined to one spot where there an item directly touches the skin on a regular basis, it sounds more like contact dermatitis to me. Any creams may exacerbate the situation. If it was a food allergy, I would expect a more widespread problem.
I would suggest, if he wears a cloth or nylon collar, get a natural leather collar with stainless steel loops rather than coated brass. Since colorful nylon and cloth seem to be the rage right now, it may be a little more difficult to find. The payoff is the leather collar will last much longer than cloth or nylon.
If it is felt a cream or ointment must be used, break open a Vitamin E capsule and rub the goo on his rash. It is odorless and if Kiya feels she must be Little Miss Nursemaid, the Vitamin E will benefit her as well.
