You can check with the IRS to see if his employer sent in a W-2.
If he is disabled, are you claiming him as a dependent? Was he employed as an employee or self-employed? Did he make more than $14,600 as an employee? If he made less than that as an employee, he may not need to file a return if that is his only income.
Can you check bank statements to see if there are deposits from a company?