If my son was born with a disability is he considered disable?

The IRS defines disabled this way:

Permanent and total disability.

You have a permanent and total disability if you can't engage in any substantial gainful activity because of your physical or mental condition. A qualified physician must certify that the condition has lasted or can be expected to last continuously for 12 months or more, or that the condition can be expected to result in death.

Substantial gainful activity.

Substantial gainful activity is the performance of significant duties over a reasonable period of time while working for pay or profit, or in work generally done for pay or profit. Full-time work (or part-time work done at your employer's convenience) in a competitive work situation for at least the minimum wage conclusively shows that you are able to engage in substantial gainful activity.

Claiming disabled has no tax benefit unless child care was necessary so that you could work that normally cuts off after age 12, but is allowed if the care is necessary for an older disabled child.

It also allows claiming a Qualified Child as a dependent beyond the age of 18 and would do nothing for a younger child.