To claim your child as a dependent, he must be unable to perform gainful work, due to a condition that is permanent or will last at least one year. (The tax definition of disabled depends on the ability to do gainful work, that is, work for wages. Many people with medical disabilities are able to work and are not disabled for tax purposes.) He must also meet the income and support tests. It is possible, for example, that a child who became disabled in the middle of the year earned too much before their injury to be claimed a dependent this year, but they might be a dependent next year.
You are not required to submit proof of disability with your tax return, but keep records for at least 3 years in case of audit. In this case, proof could be a letter from his doctor, explaining the nature of the disability and that he is unable to work, and this is expected to last more than one year or be permanent. It would also be acceptable proof if the social security administration accepts his claim, but until then a doctor's letter should be enough.
Claiming a disabled adult as a dependent will get you a $500 credit for "other dependent." It will also allow you to claim the Child and Dependent Care Credit, if the person is unable to care for themselves, and you hire someone to care for them so you can work.
