You can claim him as a dependent if you met the necessary qualifications to do so. See Who can I claim as my dependent?
Since he has already claimed himself as a dependent, you will have to print and file your return by mail.
Later in the year, the IRS will contact both of you and ask for additional information - they will decide who is qualified to claim him as a dependent.
Depending on what they decide, there will likely be collection efforts against the losing party to recover any undeserved tax refunds or stimulus payments.
