Oh my darling, there's no way to tell him without hurting him somehow. What you can do is make him understand why you can't be his girlfriend though. Whichever way you try to say it to him, he's going to get hurt because nobody and I mean nobody lever wants to get rejected.

You wanna start by telling him that you truly value the friendship that you guys have but you want to be honest with him. At that point you proceed to telling him that you only see him as a friend. You can decide to throw in a little flattery and say that although he's obviously an attractive lady, you don't want to ruin your friendship with him over something like a relationship.

Whatever the reason is that you don't want to date him, let him know so that he won't be stuck wondering why you said no to him. Honesty is always the best policy, but if you know the truth might hurt him, then you best figure out a very subtle way to tell him why you can't date him.

At this point all you can do is try and minimize his heartbreak.