It's also possibe that, as a Teen, he's had a bunch of mood swings, and they can really make a teen act up and lose friends...
If he really doesn't want to go back to his girlfriend though, maybe it's time to go with his wishes? I know it's hard when you've set your heart on a partner for your sim, but they do have a tendency to make their own wishes known, and in Sims, as in life, things don't always work out the way we plan...