Go to My Info and completely delete him. But are you sure you can't still claim him? How old is he? Is he a full time student and how much income did he have?
If he is under 19 you can claim him no matter how much he made. Or a full time student under 24. He has to file a return for his income even if you can claim him. If he qualifies as your dependent He has to check the box on his return that he can be claimed on your return.
