According to the IRS:
“To claim your child as your dependent, your child must meet either the qualifying child test or the qualifying relative test: To meet the qualifying child test, your child must be younger than you and either younger than 19 years old or be a “student” younger than 24 years old as of the end of the calendar year.”
BUT, there is an additional provision for disabled children:
To claim a disabled family member as a Qualifying Child, the person must meet the same tests to qualify as any other dependent. However, in the event that they are permanently and totally disabled, the age requirement doesn’t apply.
Now, I have no idea how to “prove” total disability for a child, so I think you should consult a CPA after he graduates next year as to how you will deal with this for future years. Or perhaps if he is able to hold a job and earn some money, he will file independently –and may not owe any taxes after he gets a job, depending on his income. So you need professional tax advice for the next few years to handle this correctly as the situation changes.