He "fled" Vietnam after the fall (liberation) of Saigon:

Mr. Hyunh is an immigrant from Vietnam. He was born sometime after 1949 ("Snow"), but he was already a young adult with a baby daughter in 1975 when the Vietnam War came to its dramatic climax.

At the conclusive battle of the war (the Fall of Saigon), the United States sent helicopters to evacuate South Vietnamese citizens. Mr. Hyunh and many others abandoned their old lives and attempted to board the last helicopter, but the pilot claimed only one more was allowed on board. Mr. Hyunh offered his daughter and she was taken aboard, never to be seen again for many years.

https://heyarnold.fandom.com/wiki/Mr._Hyunh.

So.... what was he?