He is worried about Michael and also probably a bit concerned about how the Don will take this (whether it's a success or failure) once he is well enough to be told. Assume he's had a few days to think about the whole plan. While he surely agrees with Michael that the Turk must die or another attempt will be made on Vito's life...he also knows another war is about to begin. I think he has a big knot in the pit of his stomach about the whole thing.

Yes, Tom is freaking out, and for quite a number of reason.

Apple