When you interact with him, drop MAJOR hints that you only see him like a friend. For instance, when you compliment him, say "you're such a good FRIEND", "I'm so happy to have a FRIEND like you", or something like that rather than "I don't know what I would do without you!" which could lead him on. This is pretty subtle but if he's smart enough to catch on, then he'll know.
But some guys I admit are a bit too thick in the skull to get those subtle hints. It doesn't mean that they're stupid but that they're just unobservant. If you find yourself in that situation, talk to him about other guys that you find attractive and ask him for advice about how to get close to those guys. If he reacts badly, then you can confront him about the possibility of him liking you. Just say something like "what's wrong? I really like this guy and you aren't willing to help me? What kind of a friend are you?" just to push his buttons and get him to admit things himself. Then once he admits it, tell it to him straight but pretend that you are surprise that he likes you. Say something like "oh, I didn't know. Well I'm sorry, but I've never thought of you that way....". It would hurt him but at least he'll know. Just don't be rude about it. If he seems to react sadly, then that means that he knows that you don't like him and he's feeling disappointed so your job is done and you don't have to do any actual confrontation. If your friend doesn't seem to catch the drift (which is unlikely but possible), then be more explicit on why you want to get to know __(insert name of attractive guy)__ and your potential romantic interest with him to let your friend know that you OBVIOUSLY have your eye on someone else.
However you do it, just don't be rude. There is no way to do this without hurting him, but you can avoiding hurting him more than necessary. Anyway, I wish you luck!
