Yes, you can enter in the amount they received by going to the Recovery Rebate Credit Worksheet and entering the amount your son received on Line 13. This will properly calculate the credit which is shown on Form 1040 Line 30.
You should be able to find it in the step-by-step area as well. If you go into Other Tax Situations and select Done with Other, TurboTax will ask about the third stimulus payment if they are eligible to receive any credit on their 2021 tax return. You can answer Yes to Did you receive a third stimulus? and then No to Does this match the amount on Letter(s) 6475? You can then enter in the actual amount they received ($1,400).
@cathyo21
