Based on what you're describing, he would appear to qualify for the exemption.
According to Severely disabled in the Form OR-40-N and Form OR-40-P Instructions:
You’re considered to have a severe disability if any of the following apply:
• You permanently lost the use of one or both feet.
• You permanently lost the use of both hands.
• You’re permanently blind.
• You have a permanent condition that, without special equipment or outside help, limits your ability to earn a living, maintain a household, or transport yourself.
• You’re unable to earn a living due to a permanent condition or an impairment of indefinite duration
There is a requirement for a doctor's note -- his social security disability letter apparently isn't sufficient. However, you don't need to submit it with your return, just keep with for your records. Per the section cited above:
If you have a severe disability, your physician must write a letter describing it. Keep the letter with your records in case we request a copy.
