The urgency Michael was "frantic to get Fredo in his car" for Fredo's safe passage back home because Fredo was still Michael's brother

Michael still looking out for his brother's welfare, to get Fredo safely back home even after having just learned that Fredo had betrayed Michael, nearly resulting in Michael and Kay being killed. Oh! What a brother!

Quote
Tom: Uh -- I think he got out -- he must be somewhere in New York
Mission accomplished!

Michael didn't know
1. anything about the Senate hearing then [Desert Inn]
2. nor Fredo's deeper involvement with Roth and Ola until Fredo's boathouse outburst

Hence Michael to Tom [Desert Inn]
Quote
Alright -- I want you to get in touch with him -- I know he's scared -- tell him everything's all right. Tell him I know Roth misled him -- and he didn't know they were planing to kill me