Your issue falls into the same type of scenario where stimulus checks are mistakenly being sent to deceased individuals. Mistakes in the IRS databases are causing this problem.
Hold onto the check and wait for the IRS to issue guidance on how this should be handled. The Right thing to do is to return the check received by your son. I am sure that you completed his 2018 tax return correctly by indicating on his tax return that he could be claimed as a dependent. Dependents are Not eligible for a stimulus payment.
