If he is your dependent he would qualify for the Other Dependent tax credit of $500 which should have been entered on your 2020 tax return, Form 1040 Line 19. He is not eligible for the Child Tax Credit in 2020 or in 2021 due to his age.
That depends. If he will be 18 before the end of 2021 you cannot get the child tax credit monthly payments for claiming him.
If he was 17 before the end of 2020 you do not get the child tax credit on a 2020 tax return. But you do get the $500 credit for other dependents for him. And you get the 3rd stimulus payment for any dependent you claimed on a 2020 tax return.
And...if you qualify for earned income credit you still get EIC for him.