so basically... he told you. That he likes her more.

Premises:

1. he likes you when he's with you.
2. he likes her when he's with her.
3. feelings are stronger when they are together than when you guys are together

--> to me.. seems like he likes her more.
It is true that he may change and love you more... but does not seem likely.

I would avoid being the intruder, leave this relationship you have with him. Look at it this way, if you guys will separate when you guys are at university, chances are... he won't like you as much anymore--- and he'll find another girl to like. Why be with somebody who is not sure of his feelings?

If he's from overseas... chances are he's going to go back to his girlfriend and he'll choose her over you. It's true, that we may never know... but why all the trouble?