A person is either someone's dependent or they file independently. If they qualify as a dependent, it is not up to them to decide that they wish to file as independent. They are not entitled to tax benefits that an independent person would receive, such as the stimulus.
If you choose not to claim him, you will at least probably lose a Child or Other Dependent Tax Credit. Even if you were to not claim him, he is still not entitled to claim himself. He can enter that nobody will claim him, but not that there is someone who can't claim him.
