My son was 16 until December and still in school why doesn't he qualify child tax credit

Being in school has nothing to do with the child tax credit.   If he turned 17 before the end of 2024, you do not get the child tax credit.   Instead you get the $500 credit for other dependents.  This sometimes comes as a shock to parents when their first child turns 17, but that is the way Congress wrote the law.  Remember if he was born in December---for the year he was born you got CTC for the whole year even if he was only a few weeks old.

https://www.irs.gov/help/ita/does-my-childdependent-qualify-for-the-child-tax-credit-or-the-credit-for-other-dependents

Did he turn 17 in December?  Sorry has to be under 17 on Dec 31.  But remember you got a full year Child Tax Credit when he was born in December.