If he only got the first Stimulus payment for 1200 then he gets the 2nd one for 600 on his tax return line 30. Line 30 is included and added to his refund, not subtracted.
But you should check if he really got the 2nd $600. If he claimed a missing stimulus payment on his return but the IRS took it off you have to ask the IRS. They think they already sent it to you. Did you get the 2nd round after you filed your return? Maybe it went to an account you don't remember. Or you got a check or a card. It was probably easy to miss the debit card in the mail and think it was junk mail. They have to put a trace on it.
How to put a trace on a missing 1st or 2nd Stimulus payment. IRS for Payment Issued but Lost, Stolen, Destroyed or Not Received
See question F3 here, you can expand it to see how to put a trace on it.
https://www.irs.gov/newsroom/2020-recovery-rebate-credit-topic-f-finding-the-first-and-second-econom...
