This sign can be caused by what may be a fact of human nature. It can be caused by jealousy, not having other friends, and your child's wanting to have what he has all to himself. Whatever the cause, it is a quick way for him to lose friends.
If your child is getting too possessive, you might say, for example, "I want to talk with you about you and your girlfriend. It's just something I learned along the way. Here it is. If you're too possessive, she might like it at first. After awhile, she'll want to spend time with her other friends and do stuff when you aren't around. This doesn't mean she doesn't still like you. It just means there's more to life than one relationship. Here's the problem. It works better not to be so possessive to begin with. I think if you keep it up, you might push her away. I have a couple of suggestions if you want to keep her as your girlfriend." Be careful, though. In all likelihood, the relationship will not last very long anyway. The risk is making him feel breaking up was his fault whether his being too possessive had anything to do with it or not.