How do I tell a guy that I'm not interested?

Be honest with him. Tell him how you feel. There is a great possibility that he feels the same way about you as a result of the distance. As you said, he's a great guy, and I think that he will be able to understand what is fair, for this relationship, as well as you do. As neither of your hearts are fully into that old connection you had, honesty is the best road. It will show that because you can be honest about how you feel, you can have an honest and not awkward friendship for that five weeks and beyond.

I hope you got the main idea of that.
Good luck.

I think I'm in the same situation as yours, only we didn't have a long distance relationship.

So based from personal experience, just go ahead and wing it. Delaying would only cause you more trouble and possible drama. It took me 3 weeks of contemplating whether or not it was working out. I couldn't bring myself into telling him how I feel. But I did and, well, the status is "to be continued". I couldn't think well so I told him to call me tomorrow (yes, its very recent).

So TELL HIM HOW YOU FEEL. It feels awkward, and it's so hard to initiate the conversation, but you're gonna have to do it sometime, and the sooner the better. He's a big boy, he can take it. ;)

I agree honesty is the best way to go. Be true to the way you feel. I'm sure he will understand. He might even feel the same way but someone has to put it out there. It will make your friendship stronger if you guys know how each other feels about the situation. I'm sure you guys will always remain friends and be there for each other.

I agree with being honest because you don't want to be hop-scotching around lies for 5 weeks in the summer. And then later on if you stay friends. I would just explain it how you did here, how because of the distance, it's just not the same. You've become more of penpals or something like that, and not partners, because of the distance. And coming to an understanding before he gets to you might help so that he doesn't have the wrong idea when he does get to you.