If my child starts receiving SSI for his mental status do I claim him as he is disabled

[macuser_22]

Not necessarily.  The tax code definition of disabled and the Social Security definition are not the same.

Permanently and totally disabled.   Your child is permanently and totally disabled if both of the following apply.

• He or she 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.

Also, marking disabled only allows the child to be claimed as a Qualifying Child beyond the age of 18 or to provide child care beyond the age of 13, so for a child younger than that, "disabled" makes no difference.