You don't necessarily have to "end" your friendship. You just some space, lots of space. Since you say that you are quite different from him now, it seems that you have grown apart and have little in common. He needs to hang around people who share more of his common interests.
You didn't say why he likes to hang around you so much. Does he have other friends? If not, that will be kind of tough because he is depending on you for company. Try to introduce him to others in group activities and see if he can form some new friendships there. What about clubs at school? 
Tell your friend that you and girlfriend are going to go somewhere by yourselves, and you won't be back for a long time. If he insists on coming with you, just be firm and say that you need this time alone with her. Tell him that you need some space. Be honest, but not brutal. 
If you really don't care about spending any more time with him, then you can make yourself less and less available for him to hang around with. Tell him you're busy, or what he wants to do does not interest you. It's important for him to understand that it is not because there is anything wrong with him because you don't want to shatter his self-confidence. People's interests change, and that is part of growing up. 
It's a tough situation, but your friend will figure out that you two are growing apart and that he has to go out and find other people to hang out with. It's better for the both of you because you don't want to grow in your resentment toward him. Good luck with this.