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.
That is just about the only credible explanation, although it's a long stretch. Michael was hardly the sentimental type, and, since Michael told Tom that Fredo was "weak and stupid, and this is life and death," the same logic would have led Michael not to trust Fredo with his Roth plan, if for no other reason than Fredo could have stupidly blurted it out (as he stupidly blurted out that he and Ola had been to the Superman show).
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.
Again, by telling Tom that Fredo was "weak and stupid," he was telling Tom that Fredo couldn't be trusted. At that point I think he meant trusted to run the family in his absence. So, I agree with you: at that point Michael didn't think Fredo had an
active role in the shooting. But I think Michael wasn't ruling out Fredo having done something inadvertently through his own carelessness and stupidity.
