just in case @Hal_Al 's response didn't cover the questions/ issues:
<<My understanding is I get child tax credit only if the child is under 17.>>
correct, but when you claim him, you are eligible for up to a $500 "other dependent" tax credit.
<<when he could file for himself and could benefit from AOTC.>>
yeah, but, at his income level, and the fact that he passes the 3 test questions I posted above, it is unlikely to be any AOTC benefit for him. His income would have to exceed $14,600 for the FIRST dollar of tax benefit to occur.
IMHO: Congress was smart: AOTC is really targeted at parents - and they didn't want parents who earned more than $90,000 (single) / $180,000 (joint) to circumvent that rule by not claiming their child. So they instituted those 3 rules as part of the law that I posted above which probably covers 95% of college age students - making THEM ineligible as well!
the best think to do is you claim him; he files a tax return for his income (checking the box that he CAN be claimed by someone else, which he has to do even if you decide not to claim him) and let the chips fall where they may in regard to tax credits.
