Why can't you claim him? If he is under 19 or a full time student under 24 you can still claim him. Did he file a return and claim himself? He will have to amend and you will have to mail your return this year.
Who can I claim as a dependent?
https://ttlc.intuit.com/community/credits-and-deductions/help/who-can-i-claim-as-my-dependent/00/267...
