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.
Okay, I have a lot of girl friends and I tell them always, if you want to tell a boy something negative, like that you are not interested, ALWAYS keep it factual and not emotional. I might be wrong, but ... this is what I told one of my girl friends when a guy was really hard trying to flirt with her and was just, none stop trying.
First, say you are not interested. If he doesn't get the hint, tell him why you are not interested. If he still keeps trying, then you say, okay, I give you a chance, if you have these qualities, but first I need to see those qualities. Most guys give up if they are not emotionally interested.
But you have someone emotionally interested so that would be really hard. You are going to break his hart either way, so just do, and don't drag him along by giving hints and vague rejections. I will tell you, those hints and vague rejections is what hurts the most. Just tell him, and don't try to stick around like my ex is doing now.
Again, I might be wrong on all of this. All suggestions.
You will actually be doing him a favor by telling him the truth. Getting hurt or taking it as a part of life and moving on is his choice to make. There is no other way to go about it because you do not feel about him the same way. You can try and make it easy on him by conveying it kindly.
Technically you tell a boyfriend that you do not want to be his girlfriend. To a friend, you can tell that you do not want to be his friend.
People rarely die of broken hearts.
This is a difficult step to take a friend, it has happened to me more than once that I have a friendship and it ends in a loving confusion. The best you can do is be honest with yourself and your feelings. Nobody deserves to silence what they feel, we all deserve the best version of sincerity, clear accounts keep friendships. I understand that it is easy to say, the problem is to do it. If they are really friends he will understand your feelings towards him and that there is only one friendship and should not bother him, maybe at the beginning if there is much pleasure it will be uncomfortable, but then you will feel much better with yourself and that is what should matter most to you. First your mental health. If they are really friends, he will understand. Nobody breaks the heart on purpose and less when it comes to a friendship.