here is the official IRS worksheet to determine support.
https://apps.irs.gov/app/vita/content/globalmedia/teacher/worksheet_for_determining_support_4012.pdf
Normally, the 529 is in your name listing him as the beneficiary - that is YOUR money, not his. If that is the case, YOU paid the tuition, not him.
if the result of this worksheet is he did in fact provide more than 50% of his support, then he is not your dependent as support is one of the tests of dependency. Run though your scenario here:
https://www.irs.gov/help/ita/whom-may-i-claim-as-a-dependent
