If you file jointly, assuming you are not filing only to get back tax withheld, his parents can’t claim him. You would have to file as married separately which is a disadvantaged filing status. You all would have to compare the parent’s savings with what you would save filing jointly to see what is better. Given that the child tax credit would only be $500 for them, you might save more filing jointly.
Note that to claim him they would have to be able to show that they provided more than half of his support.
