Yes you can get both the 1st & 2nd rounds on your 2020 return if you are claiming him and he is under 17 on Dec 31, 2020.
When you go through the federal review tab it will ask how much you already got if any. Or to enter your stimulus payment in TurboTax, type "stimulus" (without the quotes) in the Search box, then click the link that says "Jump to stimulus." Say no to the amounts it shows you. Then 2 boxes will open up for each round. If you didn't get one enter a 0 in the box.
After you have entered your tax information and are ready for the Federal Review, it will be the first question.
If you are trying to trigger it, you can go to Federal - Other Tax Situations, answer any questions and click Done. The next automatic selection is Federal Review and it will ask about payments received.
The Recovery rebate credit is on 1040 line 30. It either adds to your refund or reduces a tax due.
