I'm assuming you (and your spouse if married) got the full amount of the 1st and 2nd stimulus payments except for your son. payments 1 and 2 are actually based on your 2020 tax return. line 30 of 1040 should be $1100. $500 for the first stimulus and $600 for the second. I also assuming you otherwise qualify for the full credit for him (your adjusted gross income wasn't over the limit and you were entitled and did claim him as a dependent on your 2020 return)