How do you train a possessive Pomeranian ?

Have you tried training it when he was a puppy? If you haven't, that might be the problem.  If not trained early as a pup, it might not think you as its owner. Try training him how to sit and wait  and reward him when he listens to you. It could have other problems like food aggression or resource guarding, or maybe with you. Try figuring out what the problem is. And please don't use a shock collar, for it could hurt him.