Yes, the only way you can claim a 24 year old child is if that child is disabled. In addition:
- Your son must be unmarried.
- You must provide more than half of his support.
- He must have lived with you more than half of the year and
- No one else can claim him as a dependent.
The IRS defines a disabled person as one who cannot engage in any substantial gainful activity because of a physical or mental condition.
This is the same definition used by the Social Security Administration. If your son gets Social Security Disability, you can rely on that as proof of disability.
Otherwise, the question of disability is a medical question and should be posed to your son's doctors.