Well, you may have 2 issues.
1. Since you are able to claim him for 2021 I wonder if you could have claimed him for 2020. If you could have claimed him—even if you chose not to—-he should have checked the box on his return saying that someone else can claim him. In that event he would not have gotten the stimulus.
2. If he qualifies and is claimed as your dependent in 2021 you are eligible to get the $1400 stimulus for him so would report that you got $2800 and the program would give you the extra $1400 for him.
