A child must be under the age of 17 at the end of the tax year in order to get the child tax credit. If you claimed your son as a dependent on your tax return, he should have qualified you for an other dependent credit of $500. Are you saying you didn't get the other dependent credit?
If you do not get the stimulus payment for your son this year, you can claim it on your 2020 tax return which you file in early 2021 since it is actually an advance on a 2020 credit.
