I'm dating my friend's old crush but I'm going to wait to tell him.
Why are you going to wait to tell him?
Because I'm afraid he'll never talk to me again if he knows.
I understand why you feel that way, but I think it's bad to not tell your friend that you're dating his old crush. He's going to find out sooner or later, so it's better to just tell him now. It's bad to keep that kind of stuff from your friends.
Nah, I'll just wait and see.
Withholding this from your friend isn't good as you're essentially just delaying the inevitable. I think  you should reconsider and just tell him, as waiting will only make his reaction more likely to be negative.