There is no way to know who else might have claimed your dependent as a dependent on their tax return. If he qualifies to be your dependent and he filed his own return, he should have indicated on his return that he can be claimed by someone else. Ask your dependent if he indicated on his return that he can be claimed by someone else. If he did not indicate that but should have, then he will have to prepare and mail an amended return form 1040X to indicate that on his return. You would have to print and mail your return to get it accepted.
You would also have to print and mail your return to get it accepted if your dependent's father claimed him and shouldn't have claimed him. The IRS will then sort out later in the year who could rightfully claim him as a dependent.
If the dependent you claimed does not qualify to be claimed as your dependent, then you need to delete him as indicated in the answer from TurboTaxVanessa.
