If your grandson can be claimed as a dependent, regardless of who could claim him, he must check the box that says he can be claimed, even if he does not want to be claimed and even if the other person agrees not to claim him. This will disqualify him from the stimulus credit. If he fails to check the box in order to get a payment, that would be considered tax fraud.
Whether or not he can be claimed depends on his living arrangements and his sources of support. You may need to actually add up all of the costs that you paid, and all of the costs he provided for himself to make sure you get the correct answer for your situation.
