Yes, you can claim him as long as he meets the other tests. There is no age requirement for dependents if your child is disabled.
Here are the rules for a qualifying child, from Rules for Claiming Dependents on Taxes:
- "Are they related to you? The child can be your son, daughter, stepchild, eligible foster child, brother, sister, half-brother, half-sister, stepbrother, stepsister, adopted child or an offspring of any of them.
- Do they meet the age requirement? Your child must be under age 19 or, if a full-time student, under age 24. There's no age limit if your child is permanently and totally disabled.
- Do they live with you? Your child must live with you for more than half the year, but several exceptions apply.
- Do you financially support them? Your child may have a job, but they cannot provide more than half of their own support."