Was his citizenship ever restored?
I’m sure it was; according to the Telegraph article, he joined the Army Air Corps after being freed (at last) from the internment camp. Sounds like he became a career Air Force officer, serving in both Korea and Vietnam.
During my own career in the USAF, I met a few NCOs who were foreign nationals (Filipinos and even a Brit), but I never knew an officer who wasn’t a U.S. citizen. I’m guessing his citizenship was restored once he became a member of the Air Corps. As I recall, there were American volunteers who took on Canadian citizenship to fly with the RAF and their Eagle squadrons. But they regained American citizenship when they accepted commissions with the U.S. military.
Neither he nor anyone else that flew for England, or the Flying Tigers for that matter, actually “lost” their citizenship. They were put into some kind of administrative limbo so that they could go fight for another country while we were officially “neutral.” One of the Flying Tigers (Tex Hill I think) talked about this process. Once we were offically in the war, all that business went away.