The IRS is basing stimulus payments on 2019 tax return information. For people who filed early, they got their payments pretty quick. For people who did not file yet, the IRS has no good way to know how much to pay. And some people who don’t file tax returns are eligible for a stimulus payment, and the IRS created a non-filer form to say “I don’t need to file a tax return but I qualify for a payment.” If your son filed that information to get a payment and listed his child as a dependent, then he has claimed his child as a dependent and you can’t.
