Talks are ongoing, but no deal has yet been agreed upon.
Yow, 19, has three goals and two assists in 32 MLS appearances (nine starts) since his debut in 2019. The D.C United academy product has appeared in seven MLS games this season. D.C. have a club option for Yow in 2023.
Westerlo won the Belgian second tier last season and will play in the top tier this year. New York Red Bulls goalkeeper David Jensen spent the last year on loan at Westerlo, where he was the first-choice goalkeeper in their title-winning season.
Yow seemed set for a bigger role in D.C.'s first team under former coach Hernan Losada after the club traded Paul Arriola to FC Dallas, but consistent starts haven't emerged under Losada nor interim coach Chad Ashton.
The forward, who can play on either wing, was a constant call-up with the United States U-17 national team.
D.C. United have previously sent a young, rising homegrown player to Belgium before, sanctioning the transfer of Chris Durkin a few years ago to Sint-Truidense. Durkin returned to the Black-and-Red this year in a permanent deal.
D.C. are off to a slow start in 2022, with 14 points from their first 13 games (4W-7L-2D), leaving them second-from-bottom in the Eastern Conference standings.