Try checking whether or not the details in your son's tax return were updated in 2020. They should no longer indicate he can be claimed as a dependent on someone else's return. His standard deduction would be impacted if he is not filing as single.
Secondly, if any of your information was entered in his return regarding "Kiddie Tax" calculations, that may be an issue. The higher parent's tax rate may be charged on the child's return based on his age and whether or not there was unearned income greater than $2,200 if the box is checked that he can be claimed as a dependent on another return.
For more information, see:
@gchmelewski
