The standard rule is that the qualifying "child" must be under the age of 19 at the end of the tax year. That is, the younger person can be 18 years and 364 days old on December 31 and the age requirement is met. If he is still your qualifying child, then you can claim him.
He can file his own return to get back any withholding that he is due. When he does that, he must mark the option that someone else can claim him as a dependent.
There is limit to the amount of money that he can claim if he is under 19 on December 31, 2021.
