You lost the child tax credit in the year that he turned 17. There are no exceptions to the rule. You can still claim him as a dependent, but you get the $500 credit for other dependents instead of larger child tax credit.
https://ttlc.intuit.com/questions/1900923-what-is-the-child-tax-credit
https://ttlc.intuit.com/questions/4499708-what-is-the-500-credit-for-other-dependents-family-tax-cre....
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