Was he a full time student? Sorry if he is over18 and not a full time student you can't claim anyone if they made 4,300 or more.
Who can you claim
https://ttlc.intuit.com/community/credits-and-deductions/help/who-can-i-claim-as-my-dependent/01/267...
See IRS Pub 501 starting on page 10 bottom Dependents and Worksheet 2 page 15 for determining support,
https://www.irs.gov/pub/irs-pdf/p501.pdf
