My point is the FoB could have come back to life shortly after he died. Also I remember reading a post somewhere along the line that in Torchwood Jack mentions that he does in fact age, Just really really slowly. So that would account for the change of his body. And he is definitely the FoB, I forget the episode name but at the end of it jack says he was once called "The Face of Boe" The Doctor and Martha Jones then look at each other in realization. Suddenly it makes sense why FoB was "an old friend" and wanted to help the doctor with The Master.