The IRS rule is that he cannot claim the stimulus if someone else can claim him as a dependent. In this case, you can claim him as a dependent and he is not eligible for the stimulus. He can file his own return to get any refund he may be due from work but should check the box that someone else can claim him.
If a person can be claimed as a dependent by someone else, then they are not eligible for the stimulus even if that person does not claim them.
