I'm going to tell my best man for my wedding that he wasn't my first choice.
Why would you tell him that?
Because it is my wedding and I want it to be perfect!
You should keep that to yourself. Just get on with the wedding and enjoy your day.
I think I'll tell him during the rehearsal dinner.
It's a bad idea to go talking about all your decision making to your best man with regards to picking him. He could get his feelings hurt if you spring this on him.