Watch - just tell him straight. Don't dress it up to spare feelings, as nasty as that sounds. You need to be direct and don't leave any room for debate. A simple: whilst I have been away, I have been doing some thinking. We are two very different people and I know we don't have a future together. I am certain of my decision and am ending this relationship. All the best.
Then don't respond to any further communication. Break ups are horrible but he will move on and you may even help him to grow up. Just be careful not to encourage him in any way to think there is any chance.
As for leaving you hanging 
okay I will spill 
So background. Met PMF (platonic male friend) when I started my college course end of August. We made friends straight away partly because we were at the time, the only smokers in the class and because we were at the time, the two oldest. We are also pretty well matched in what we know regarding the course we are doing so when we had to be in a group we paired up. After college he would come back to mine before going to work and muck about with the kids, do assignments. He never flirted etc and I was still with S so we were just college mates, nothing more.
Fast forward to last week. We were out with two of my friends at a karaoke night getting drunk, dancing and singing. He pulled me up for a dance and whilst I was laughing my head off at his bizarre dancing he just leaned down and kissed me. I was 
and 
and 
for about five seconds then kissed him back. We even got a round of applause from the bar haha!
He later told me he had a crush on me from the very first day of college before the karaoke bar I had never really thought of him that way, because I was still with someone and because he is five years younger than me but I realised I do actually like him and we have started tentatively seeing each other on the basis of nothing serious, just...fun. The end.