Disability is not an exemption from filing taxes if he has taxable income. The 1099-R is probably taxable, depending on what kind of account it is, and the social security might be taxable if his other taxable income is over the IRS limits for social security.
If he is not competent to prepare and sign his return, someone can do it for him if he has a court designated guardian or if he gave someone a durable power of attorney while he was still competent. If not, someone will need to go to court to be appointed his personal representative.
