Honestly the most tragic part of this Drama is that he's actually right. I don't think he's worried about her not loving him, he's worried about him stifling her. And it's understandable, she's lived an incredibly isolated, sheltered life, and then he was her first contact outside of that world and the one who she began even going to school for. It is actually a good thing that she branches out and makes friends outside of him.
Of course the tragic part is simply that he's clearly not the sort of person to really get in the way of that even if he's in her life. But then again their Middle Schoolers. He's not going to be able to see relations with the nuance needed to understand that.