Can I claim not getting stimulus for my son who I took absolute full custody of my son in May 2020?

[jacobconklin2020]

His mom doesn't pay child support and kept his stimulus check even though I've fully had him since may.

[SusanY1]

Probably.  When you file your 2020 taxes, be sure that your son is listed as a dependent in the Personal Info section of the program.  Prepare your return as usual, and at the end be sure to indicate the amount of the payments that you did receive.  TurboTax will pick up on the difference between that amount and the amount you are entitled do based on the dependent information and add that amount to Line 30 of your 1040.  This will increase your refund or decrease your balance due.  

[NCperson]

if your son is your dependent on your 2020 tax return, and under the age of 17, it'll work out on the 2020 tac return.  Just look on line 30 of Form 1040 and any additional money due you will be there.  if not, post back and we can work through it.