If the child lived with you for more than 50% of the year, then you can qualify as the 'custodial' parent to take the Dependent Care Expense Credit, even if your grandson is not your dependent. No one else can claim the Dependent Care Expense Credit for your grandson.
Otherwise, he must be your dependent to qualify for the credit.
If you qualify, TurboTax can handle this. You'll enter him as a 'dependent' (even though he won't qualify) and then you'll be able to select his name under the Dependent Care Expense Credit questions.
Search "Dependent Care" in the top right search bar to get to these questions.
