On the one hand, he was extremely dedicated to achieving unconditional surrender. On the other hand, he was more willing to work with Stalin (afaik) than Truman, and Byrnes would never have become Secretary of State. Would FDR living to the end of WW2 have changed the outcome of the war with Japan?
I don't think so. Japan was weeks away from surrendering, so you don't even need to have a president who's unwilling to use the bomb - just one who will wait a little longer before deciding to do it and will end up not having to.