Under the tax laws, a person who can be claimed as a dependent must check the box that says they can be claimed, even if the person who can claim them doesn’t want to.
Your 25-year-old child can only be claimed as a dependent by you if their income is less than $4200 per year and you provide more than half of their total living support. If those conditions are true, then you can claim your child, and if you change things around so that your child appears to qualify for a payment, it would be fraudulent.
If you claimed him/her and/or he checked the box, on his return, that said he can be claimed as a dependent, he will NOT receive the stimulus payment.
What is not clear is what will happen if you and he file amended returns to unclaim him and uncheck the box. As you might imagine, many people have asked that question and so far a clear answer has not been received. For sure, it will be a while (months) before he gets the stimulus.
But, if he doesn't get it in 2020, he will get it in 2021, if he is still not a dependent in 2020.
"In essence, the stimulus check acts as an advance of your 2020 income tax refund. This means when you prepare your 2020 income tax return, there will be a line to include the section 6428 credit. The credit on your 2020 return is subtracted by any amount received as a stimulus check in 2020. If the amount you received as a stimulus check is less than the credit you are due, the difference will be included as part of your 2020 refund. If you have been overpaid by receiving the stimulus check, however, you will not be required to return any excess amount".