Penolpe, this is about his insecurities not about your outgoing behaviors. He needs to trust you and believe in you otherwise the relationship is doomed.
You can reassure him that you love him and are not interested in anyone else but if he can't accept what you are telling him, I don't know how he's going to handle things when you go away to school. You can tell him you'll stay in touch with him, you'll call, you'll be back on long weekends, holidays, breaks, etc. You'll write, send cards, etc. And he can do the same.
If no matter what you do to reassure him doesn't help, then maybe its time to call it quits and move on. You can't change him, you can only change how you react to the behavior.