He can be your dependent if:
(1) He earned less than $4,050 in 2017.
(2) You provided more than half his support in 2017.
He is considered a qualifying relative in this case, as opposed to a qualifying child, which has different rules, one of which is the age rule to which you are referring.
See the following worksheet to determine if you provided more than half his support. In particular, his reliance on loans may disqualify you.
https://apps.irs.gov/app/vita/content/globalmedia/teacher/worksheet_for_determining_support_4012.pdf
