The real issue here is whether your son qualified to be claimed as your dependent. It doesn't matter if you actually claimed him or not.
If he qualified to be claimed as your dependent, then he had to indicate that on his tax return by checking the box indicating that "someone can claim you as a dependent". If he qualified to be claimed and failed to check the box, then it's his return that's incorrect, not yours.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
