First, did YOU take him as a dependent on your 2018 return?? Think carefully, because that would be an immediate disqualification. That’s the first place they scanned for SS numbers when the program started in April.
Second, did he file his own tax return in 2018? Has he filed his own tax return for 2019 (must be done this week!).
If you declared him in 2018, but not in 2019 –and if he filed his own tax return in 2019, then NEXT YEAR when he files his 2020 tax return in April 2021, he may be able to claim a credit for the stimulus that he did not receive.
Now, if you didn’t declare him as a dependent AND if he didn’t file in 2018 there is one more possible thing to do. Have him fill out this form immediately: