Most Helpful Guy
First off - You seem like a very fantastic person with character. That I already like, as it shows that you do care for the other persons feelings when it comes to situations of "bad news" coming their way.
Secondly - To let you know ahead of time, someone's feelings is going to get bummed on regardless the situation. Either you drag this on and it affects you even more or you let him know straight on and get it done quicker. It's definitely not an easy cookie to swallow.
The best thing that you can do is to tell him straight out how you feel and probably in person because you can actually tell how the person is after they hear the news. Either they will react in a poor manner (no friends zone) or be proactive and mature (friend zone) about it. But after seeing his reaction, you'll know what to do because some nice guys do become super salty and flips a 180 in their demeanor, those are the ones you'll have to be more alert on. Have an honest conversation. If this was flipped on you, ask yourself, "How would you like the news to be broken to you?" Flipping your perspective.
It seems like you have no ill will with this guy, so just break it to him. If you guys didn't click, then you guys didn't click. Watch out if he comes with a clinger rebuttal of "let just give it some time", but use your intuition. If you know he's not for you, then that's ok.
Tell him first how you think about him, being mr. Nice guy and all, and then go for the truth on why you think you don't click with him. And friend zone after is to your discretion. Don't feel bad, it is what it is. Just do the right thing and learn from it. Hope this helps :)
Most Helpful Girl
Just be honest and tell him everything. Not in parts but the whole story. Yes it will hurt him a bit but trust me it's better than listening to parts and then questioning yourself on what you did wrong. Also don't directly say you want to be friends because that will give him some hope to try again. Just ask him if he can be around you despite knowing you don't want a relationship? I think you will get your answer and he will get less hurt.