I think when you get one or the other, they check to see if you are eligible for both. But SSI can vary some by state (the Federal portion is the same nationwide but some states add more of their own money to it) so I would check with your state. But here's the Government's brochure on it, to see if he's eligible, etc.:

http://www.socialsecurity.gov/pubs/EN-05-11000.pdf