Does he like me or not? and how will i reject him if he does?

ok so i was at my friend's birthday party and my guy best friend and i hung out pretty much most of the night..anyways there was a really big inflatable slide thing and we were pretty much up there the whole night and every time 1 of my friends came up and started talking to me he tried to get them to go away and when i tried to follow my friends he didn't want me to leave..he's a really nice guy and i can't tell if he likes me..does he? if he does, and ends up asking me out, i'm probably going to tell him no because he's like a brother to me and i don't want to ruin our friendship..how should i tell him i dont think we should date besides the whole ruin the friendship thing?