Note that the OP's grandfather was not a US citizen, but a refugee from a declared enemy during a time of war. There's a huge difference between the two. That fact that he was even allowed to practice medicine is amazing.
In odd bureaucracy, he DID get a doctor's special gas ration anyway. (He had a good institutional job in the Kaiser shipyards.)
Come to think of it, that's another reminder that in those days, medicine --> home visits.
This is all documented in exrutiating detail in Rhodes' series on the making if both the atomic and hydrogen bombs.