If his mental health issues qualify him for Social Security Disability Insurance benefits, those issues probably qualify as a 'disability'.
Social Security disability benefits may be taxable if you have other income that puts you over a certain threshold. However, the majority of recipients do not have to pay taxes on their benefits because most people who meet the strict criteria to qualify for the program have little or no additional income.
Enter his SSA-1099 in the Social Security (SSA-199) interview of TurboTax. The program will take his other income (if any) and then determine if the SSDI benefits are taxable.