I recently noticed in the book. when they talk about maybe someone other than Michael taking out Sollozzo, they mention:

Sonny, but he has to run the Family;
Freddy, but he's in shock;
Tessio or Clemenza, but Sollozzo would be wise to them;
Some other button man, ditto;
Some young, tough kid, but that would be like calling a minor leaguer up to pitch in the Series;
So it has to be Michael.

Tom's in the middle of the conversation, he owes Vito more than anyone, but there's not even a word that he might do this.