I'm sorry to hear about your situation.
But i'm going to give it to you straight - at least, in my experiance. For 13 years he's been at school, most likely in one village for his entire life, seeing the same people every day, going to the same school, same lessons and same shops. When you go to university, you're suddenly surrounded by people from all over the country - hell, all over the world. You're doing the one subject you (should) love, and you're living on with all your best mates (or who will most likely become your best mates). Talking to my brother, it's like an awakening, a bridge between being a child and an adult, like an entire new world has opened up to you that, while aware of it's presence before, it was something "other people" did.
He's changed. People change when they do to uni. My brother changed considerably. But when he comes back in Summer, does he still see his old friends? Of course he does. And they've changed too, but they get on better than ever. Changed doesn't mean being completely different. He'll have different inspirations, different tastes and a different outlook, but ultimately he's still the same person. However, you haven't changed at all. When you sees you, or thinks of you (and i'm sure he does think of you), he's reminded of his life of going to the same school everyday with the same people, in that "smaller" world.
This, i'm sure, sounds horribly condescending, insensitive and patronising, but i'm in the same situation too. Well, i'm not, but i'm in Y13, in that dreary monotonous stage before you get to adulthood, and i can only imagine what the idea of revisiting this must be like for a person who's gone out there, and done things.
My honest opinion? It won't work. At least, not until you've both finished uni - assuming you are going. He needs time to change, and what he doens't need is a person from his past trying to keep him the way he was - and i'm sure it doesnt help that you're a person he loves (and i'm sure he does).
Good luck!