There are techniques to deal with possessiveness.

Now if only I can remember the name of that wonderful man who has great dog training advice. I will dig it up. Meanwhile...

The idea is that you give your puppy de-sensitivity training for possessiveness, making sure that he knows that he does not have ownership of *things*. Of course, this is easier when it involves humans than another dog. This might be a case for a serious dog psychologist.

Since he's young, there is every chance that this can be solved. Better now than later, when he is really set in his ways.