That's a shame, hon. It looked so positive for a while there.

Maybe go for the honest approach? Say you're very fond of him but it's not working out as the kind of long-term relationship you want. The 'we'll do better as friends than lovers' thing is a cliche but it's often true. Keep it low-key with no raised voices or accusations and no blame attached would be best if you do want to stay friends with him.

Good luck. *hugs*