I certified a young boy with Down's Syndrome this past June. I can't comment on his level of cognitive disability because I have not worked with enough persons with Down's to base any opinion on. He prepped for a trip to the Keys for over 14 months (with other trip participants) by participating in discover scuba events and finally confined water sessions. He was a joy to work with, but for a while I didn't know if he'd be safe in the deep end of a pool, let alone open water. The more experience he got in the pool, though, the better he became.
As someone else stated in this thread, it took a fair amount of one on one attention. Working with him in a group scenario, while it gave him experience, was not really clicking. Just a few hours of one on one got him to turn the corner on holding his breath and his comfort level shot way up. I would not suggested one on one sessions from start to finish, I think that having people to interact with underwater was invaluable... we just needed the extra time and attention to ensure he'd be safe in OW.
In the end, he is currently certified to dive with 3 (qualified) buddies, his dad will most likely pursue scuba certification and I truly believe that if he continues to take advantage of dive opportunities (which he has since going to the Keys), he will master all of the OW skills and be able to render assistance to dive buddies, should the need arise. That is probably a few years down the road, but he'll be encouraged to keep diving.
I think that as far as finding information on training a person with Down's to scuba dive, you'll be hard pressed to locate any. Your best bet is to find a very patient instructor, do at least 2 discover scuba sessions and then feel your way through the process. I can put you in contact with my student's swim coach, who helps coordinate the program this student participated in, who will probably be a wealth of information. I can also give you more detailed information on how his training progressed.