Tab Tan Rouxin of Johor finished second with 1:14.
Hoe Yean won the men's 400m freestyle final race in 3:50.
Despite his early exit after failing to shine in both events that he took part in, Welson was delighted to have set a personal best at the Tokyo Olympics.
The tournament will mark the first time that South Sudan will field a competitive team.
Ia selepas Welson sekadar mencatat masa 1:49.
Another American swimmer Lydia Jacoby was second fastest with 1:05.