So how do you know that he likes you? Has he actually told you, did you hear it from someone, or are you just assuming that he does? Because for each one you have to take different actions.
So if he actually told you straight up that he likes you, then get him alone and talk about it. I will tell you this now that there is no way to tel him that you don't like him without hurting him in some form. But you can reduce the hurt or at least speed up his healing process if you tell him the right things. Remember to make it short and simple and don't add too much fluff. But remember to be considerate of his feelings and assure him that it's not his fault. You should watch this video or videos similar to it. It's actually quite helpful.
If he hasn't told you yet, then just pretend that you don't know anything about it. If people bring it up, just shrug it off. Don't make it a big deal until he confronts you about it. Once he does, take the following steps above. If you make a big deal out of it before actually knowing what's going on, you might make some mistaken assumptions that could end up disastrous to your friendship.
I hope this helps.