Third stimulus checks received but now dependant

Was your son receiving unemployment during 2021?   If so, that income counts toward the limit of $4300 he can receive---unless he is also a full-time student under the age of 24 you will not be able to claim him as a dependent if he had over $4300 of income.  The only income that would not "count" is Social Security.   

If in fact you can claim him, then through a loophole left by Congress, he does not have to pay back the $1400 he received last year, and you do get to receive it as a recovery rebate credit if he can be claimed as your dependent for 2021.   You do not enter anything about the $1400 your son received on your own tax return.

WHO CAN I CLAIM AS A DEPENDENT?

You can claim a child, relative, friend, or fiancé (etc.) as a dependent on your 2021 taxes as long as they meet the following requirements:

Qualifying child

• They're related to you.
• They aren't claimed as a dependent by someone else.
• They're a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
• They aren’t filing a joint return with their spouse.
• They're under the age of 19 (or 24 for full-time students).
• No age limit for permanently and totally disabled children.
• They lived with you for more than half the year (exceptions apply).
• They didn't provide more than half of their own support for the year.

Qualifying relative

• They don't have to be related to you (despite the name).
• They aren't claimed as a dependent by someone else.
• They're a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
• They aren’t filing a joint return with their spouse.
• They lived with you the entire year (exceptions apply).
• They made less than $4,300 in 2021.
• You provided more than half of their financial support.

When you add someone as a dependent, we'll ask a series of questions to make sure you can claim them. There may be other tax benefits you can get when you claim a dependent

Yes. If you were issued the full amount of the third Economic Impact Payment, you won't be eligible to claim the 2021 Recovery Rebate Credit.

Per the IRS, you were issued the full amount of the Recovery Rebate Credit if the third Economic Impact Payment was $1,400 ($2,800 if married filing jointly for 2021) plus $1,400 for the number of qualifying dependents you had in 2021.

2021 Recovery Rebate Credit — Topic E: Calculating the 2021 Recovery Rebate Credit