I had a thought, have you had the dog since he was a puppy, or could he have had a bad experience involving being grabbed by the collar? Perhaps it is the actual act of reaching towards his collar that causes a defensive/alarm reaction? When I got my golden retriever Tommy, he was reluctant/defensive about anyone taking hold of his collar, we think because he was grabbed by his collar to be chained up or abused by the people who had him before. He didn't show aggression but when we would reach for his collar, he'd flop on his back with his tail tucked or jerk his head away or even mouth/lick the hand reaching for his collar.
I think determining what you think is the cause would help you in learning how best to handle it.
Mom to Raven, and retired racing greyhounds Rudy and Cole
Missing always: Tasha & Tommy, at the Rainbow Bridge