he's 100% disabled by the service
Then he has a dd214 issued to him by the military that explicitly states he is discharged for/with 100% disablity rating. You only need that, if the IRS asks for it. I would also expect he is receiving disability pay from the VA that is 100% tax free.
Do note that he will ***NOT*** receive any kind of tax reporting document from the VA in any way, shape or form since the disability pay is tax free. It does not have to be reported on *ANY* tax return, ever. That disability pay also does "NOT" count towards any earned income maximum either. So if all he receives in a tax year is that tax free disability pay, then he has ZERO DOLLARS of earned income.
