Income from another Country

My son started working in S. Korea on May 1, 2021 and will continue until July 31, 2022 at which time he will return to the U.S.

His employer filed his taxes in S. Korea, on his behalf.  

 

We received a statement of income and taxes paid on that income from his employer.

Does he have to include this income in his 2021 U.S. tax return?  If so, how?

Does this mean he will also pay U.S. taxes on this foreign income?