What do you think about me telling my best friends secret to someone?
Why would you do that? What are you going to reveal?
I'm going to tell everyone he's gay.
It is wrong to tell your friends secret. Outing someone can be dangerous for them and cause harm. It is not fair to them to not know what is coming.
How can it really be that bad? He's going to be gay no matter what. Whether it's not or later, nothing changes that.
I think you aren't realizing the potential emotional damage you could cause by outing him and even worse the possible physical abuse he could endure if whoever you tell doesn't accept him. Coming out is a really big deal and it should be when he's ready, not when you feel like it. He was willing to come out to you and you should be both grateful he told you and keep it a secret.