The IRS is right. If you are claiming a child on your 2020 return you can get both the 500+600 amounts for him even if you ex got the payments based on her 2019 return. That is a loophole in the IRS. You can both get the Stimulus payments.
You should not need to override your tax return. If should automatically give it to you if you qualify. It will be on line 30. Unless you don't qualify for another reason like if he turned 17 in 2020 or your income is too high.
