It's not about how to tell him.
It's just that you have to tell him.
I would recommend being direct. Men tend to communicate directly more. Don't hint. Don't say “we should just be friends” don't say how good of a time you had. Because being indirect like that, while is nice and it does soften the blow a bit, it also creates an opportunity for him to look for hidden meaning, trying to find how there might be a chance.
Better to just be direct, “I have to tell you this, I don't want to lead you on. I don't have romantic feelings for you. I feel it's better to tell you this out of respect to you and your time. I'm sorry”
You cant control how someone responds to this situation, but the right thing to do is to let him know asap. And you have to find comfort knowing you did the right thing.
He will either take it well or he won't, the HOW it was said will have little to do with how he takes it. And how he responds is part of his set of challenges.
Don't feel guilty for leading him on before. You cant change that (if that's actually what happened) what you can change is not continuing to do so.