I think like most of the guys here have been saying. You need to tell him as soon as possible. Don't give him a BS answer either. Be open and honest about how you feel. He will respect you much more for that, than if you just try and ignore him or act mean to him so that he will get over you.
If that's the way you feel about him, chances are that he probably feels like something isn't right as well. I don't think it will hit him out of the blue but at the end of the day you need to tell me ASAP for him sake and for yours.
If you do it in a kind manner, maybe you guys can be good friends afterwards?
Please just tell him the truth and do it upfront so he doesn't get left confused (some guys are more intuitive than you think and can tell when you are lying) or wastes his time. Listen to the guys on this one, we know he it feels to be a guy and how it would feel.
Depending on how long you've been together I highly doubt there's a way you can break up with him without hurting his feelings, the best thing you can do is be truthful with him and let him down gently.
Just tell him because you are only making it worse by keeping the relationship going.
Just explain to him that you think you might be better off alone right now. Don't use the "It's not you.. It's me" line, but just tell him you've got too much going on in your world and in your head, to focus your attention on anyone else right now.
I had to tell one of my ex's that I was too selfish to commit my time and affection to him, at that point in my life. It was the truth, and it didn't hurt his feelings. that bad.