I know he lived until 1980.
He was such a great character - he seemed like a great side kick kind of guy. It would've been nice to see his character get closure after John Basilone died.
He did loose his 2 best war buddies. I'm sure there was a huge memorial service for John, and JP was most likely there.
This wiki barely says anything about him after the war:
http://thepacific.wikia.com/wiki/J.P_Morgan
In an interview, Joshua Bitton (who played Morgan) had this to say -
"Nobody is sure what happened to him. There are a couple of rumors...One is that he sent his poker winnings home to his wife Katie and when he came home from war, there was a Dear John letter for JP and that she'd taken the money and run off with another man. So, JP hopped on his motorcycle, grabbed his 45 and vowed to kill both of them, but he was hit by a truck and killed.
Another rumor is that he lived till he was 80, so no one is really sure what happened to him."