Its a difficult situation, I dont think it can be dismissed as 'something these dogs do' as biting is a very serious problem and shouldnt be accepted from any dog. I know a lot of dogs dont like to have their collars grabbed as its their vulnerable area (dogs fighting will go for the neck, its why they have so much scruff for protection). With you saying he's treated like a prince it may be a dominance thing, if he thinks he's pack leader in his mind you have no right to grab his collar so he could be showing you in the only way he can, biting! It could also be that he isn't getting the exercise and mental stimulation he needs, I know you said he gets lots of exercise but they need a huge amount of both free running and mental activities. I have never had a collie as even though im at home all day I dont think I could provide the massive amount of stimulation they need. We see plenty of collies out walking our dog and when I worked at the vets we saw lots of collies and id say easily 50% of them had aggression problems and it often stemmed from being bored and frustrated. I would get him checked over by a vet to make sure it isnt a medical problem then discuss with them about a referral to a dog behaviourist if they recommend it. If a dog learns that biting is ok it will continue to do so as unfortunately they learn that biting gets the desired effect i.e. you let go of his collar.