My son was born with Tetralogy of Fallot, so I know what you are going through. I wish you well.
I certainly hope that your son does not meet the IRS definition of Totally and Permanently Disabled, which is:
"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."
In truth, as long as a child is under age 13, it doesn't matter whether you indicate disabled or not. There is no special tax treatment for a child under 13 just because that child is determined to be disabled.