You can still claim him as a dependent on your return (assuming you meet all the qualifications). However, for the Child Tax Credit the rules are different then the general rules for a dependent. If your child turned 17 at any time during 2016, then you will not receive the child tax credit - even if it is only for one day of the year.
https://www.irs.gov/uac/ten-facts-about-the-child-tax-credit
