Someone told him a secret and I just wish he'd let me in on it.
Why do you want to know the secret he was told?
I really just want to know what it is.
Wanting to know is fine but you should avoid pressuring him into telling you. If you do he likely wont ever feel comfortable telling you sensitive things in the future because you tried to force it out of him.
I'm seeing him Friday. I'll ask about it then.
If you're not meant to hear the secret, then try to forget about it. Secrets are earned through trust, so show how faithful you can be and your friends will let you in on more clandestine information.