in response to MN homebuyer... Go through your local HUD center. There is a little known fact of the section 8 housing voucher: it can also be used to PURCHASE a home.
Also Habitat for Humanity, Homesite, USDA Rural Development programs, NACA home ownership. The USDA programs are for low income folks.
As for the proof letter you need, look through his paperwork. he should have received a letter from SSI that stated he had qualified and under what conditions: if he is perm disabled, the letter will state this. You can also ask for a copy of the determination/evaluation paperwork. Did he get evaluated and declared disabled by a Dr of SSI's choosing? Then there is an Evaluation Recommendation letter in his SSI file somewhere and he has a right to a complete copy of his file including that letter. Go sit in the SSI office at 8 am and talk with someone...yes, I know, it's going to be about 4 hours of waiting, but if it gets you that house then it was worth the wait. That permanently disabled recommendation form the Dr is what those people are looking for. That with the original letter that stated his disability should do the trick. Good luck.