What can I do?
Most Helpful Guy
First of all, you should have notify him before dating the new guy. I think this is your fault that causes the awkwardness of the current situation. If you told him that would motivated him to take some action.
But now, PLEASE be direct to him. DON'T hurt him ANYMORE by being indirect. If I were him, I would like to be told directly about the situation. And if you want to do a favor for your best friend, tell him about the plenty chances you gave him and point out that he didn't use them, so you found out he doesn't like to go for more.
1) you made him aware of his miss-behavior; maybe this is his way of expressing interest and he thinks doing fine, but obviously it is wrong and a good friend is the one who give the feedback to him. With thanks to you he'll be more brave about the next girl whom he would attracted to.
2) He'll understand his fault and won't blame you for everything. (except the "not-telling")
3) No one knows about the future! maybe some day you two get back to each other. Knowing that you both were attracted to each other in some time, could be great at that possible future! (Oh! I'm sorry if this doesn't seems good to you, because you are already happy with someone else, but be realistic.)