For me, I believe that Mike didn't beleive Fredo was the traitor until Superman, his utter abhorrence when he hears Fredo say take Ola took him to see the show is far to agonised for someone who suspected all along.

I think he tells Fredo because he's his brother, that telling him can't do any harm and it allows him to share what he knows with someone he thought loved him. You can see at the start 'I can't control her Mikey' 'You're my brother Fredo, you don't have to apologise to me' that there is a bond between the two, at least on Michael's part.

If he suspected Fredo, he'd have told Tom he suspected Fredo when they talk after the shooting, only Rocco and Neri's name were mentioned, not Fredo's. Tom being the only man Michael can absolutely trust would be informed would he not?

He couldn't rule it out of course, but I think Fredo was last on his list of suspects.