A complete list of the requirements for a qualifying child for the EITC is listed below.
At that age, your son must be considered "permanently and totally disabled" to qualify as a "qualifying child." The IRS describes this condition as
"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."
It's not necessary (and mostly pointless) to submit medical documents with your return, but it's likely the IRS will contact you at some point for proof that your son meets this standard. I cannot say whether the letters you have now will satisfy the IRS.
Also, keep in mind that if your son doesn't meet this test, he may still qualify as your dependent under another test. But for EITC purposes he must be a qualifying child.
https://ttlc.intuit.com/replies/4835245
