If he was not eligible for the stimulus (Economic Impact Payment), return the payment.
Go to this IRS website for how to return the payment - https://www.irs.gov/coronavirus/economic-impact-payment-information-center#returning
What is preventing you from claiming him as a dependent?
You claim someone as a dependent in the My Info section of the online TurboTax program.
Since the child is age 18, you should be able to claim him as your dependent under the Qualifying Child rules if he meets all the requirements.
To be a Qualifying Child -
1. The child must be your son, daughter, stepchild, foster child, brother, sister, half brother, half sister, stepbrother, stepsister, or a descendant of any of them.
2. The child must be (a) under age 19 at the end of the year, (b) under age 24 at the end of the year and a full-time student or (c) any age and permanently and totally disabled.
3. The child must have lived with you for more than half of the year. Temporary absences while away at college are considered living with you.
4. The child must not have provided more than half of his or her own support for the year.
5. If the child meets the rules to be a qualifying child of more than one person, you must be the person entitled to claim the child as a qualifying child.
6. The child must be a U.S. citizen or U.S., Canada or Mexico resident for some portion of the year.
7. The child must be younger than you unless disabled.
Since you are paying extra for CPA support using one of the TurboTax Live editions, see this TurboTax support FAQ for contacting a Tax Expert - https://ttlc.intuit.com/community/expert-review/help/how-do-i-connect-with-a-tax-expert-in-turbotax-live/01/27282
If he can be claimed as a dependent under the rules, then you can claim him. If you had not yet filed your tax return, he could have filled out a “non-filer“ report to claim a stimulus payment as someone who does not file a tax return. However, this would have been improper on his part. If you are trying to file a tax return now, and you are blocked from claiming him as a dependent because of this non-filer report, you will need to print your return and file by mail. Your son you should return his payment, and the two of you need to have a discussion about honesty.