When entering him as a dependent you can select disabled. However, if the child is under the age 13 it provides no tax benefit to you by doing so.
If the child is older than 12 and disabled you can receive Child and Dependent Care tax credits if you have someone care for him while you work. And when the child is age 19 or older and disabled you can claim him as a dependent under the Qualifying Child rules instead of the Qualifying Relative rules.
IRS definition of disabled -
Permanently and totally disabled. Your child is permanently and totally disabled if both of the following apply.
• Your child can't engage in any substantial gainful activity because of a physical or mental condition.
• A doctor determines the condition has lasted or can be expected to last continuously for at least a year or can lead to death.
