If you did not claim the child on your return you can amend and add him. The parent (or someone) can claim him no matter how much he makes if he is under 19 or under 24 and is a full time student.
How to Amend
https://ttlc.intuit.com/community/amending/help/how-to-amend-change-or-correct-a-return-you-already-...
