I'm not going to tell my coworker I already got the promotion he wanted
Why don't you want to tell him?
There is no reason to tell him right now. If I tell him, he will be so mad at me
That's kind of you to want to spare him that.  It's good to want to avoid hurting people's feelings, especially when it's something like this which sounds like a major disappointment for the guy.  Hopefully you find a tactful way to inform him!
Yeah, I'll let him know in 2 weeks.
That's good to hear. It's great that you have some time to figure out the best way to tell him so that you don't hurt his feelings.