Something may have happened while his collar was on that he now associates bad things with the collar. Make sure he has no injuries to his neck or any ear infection.
You can desensitize him to the collar by gradually teaching him it's a good thing. Start by showing it to him and giving him treats. Let him sniff and check it out, and praise and reward him when he does. Also start reaching over his head (without the collar, just your hand) and giving treats with your other hand. If you feed kibble, feed one by one, reach towards his head, give kibble. Touch his neck, give kibble. Get him really used to you touching his collar area. When he's comfortable with you touching his neck and when he's intentionally nosing the collar to get treats, then slowly put it on, giving treats at the same time.
It could be the snap he doesn't like-try a buckle collar.