...and the fact that he was Sonny's choice for brother, not Michael's. Attest the way he practically wept when Michael said, "You're my brother," and Tom said, "I always wanted to be considered a brother by you"--a surefire indication that he doubted it previously.

I don't think Michael believed Tom was in with Fredo. And, in addition to needing Tom as his lawyer, Michael probably sensed that Tom's relationship with Fredo would help Michael get what he wanted from Fredo--as in, "I wanna talk to Fredo" after the Senate bombshell.