Did you enter him as your dependent Child or did you select Other for him? Have you entered any earned income from work on your tax return?
Did you pay someone to take care of him so you could work? You have to enter your W-2 or self-employment income first, before you try to work on the childcare credit.
Was he under the age of 13 when the care was given? That is the age limit for dependent care credit.
1. He should be listed as a grandchild, not an "other" dependent.
2. Regardless of legal custody, he must live in your home more than half the nights of the year. If you are asked, select "7 months" or longer, since the program takes "6 months" to mean exactly half the year, and half the year is not more than half. If he lived in your home less than half the year, he is not eligible for the dependent care credit.
3. The child must be under age 13 when the care was provided. Double check the birthdate.
4. To claim the credit, you (and your spouse if you are married) must have income earned from working. That is a W-2 job or net profit on Schedule C. If you don't work, or if you work but one of you has a schedule C with a net loss, then you don't qualify for the credit.