So sorry about your beloved Westie. They are such cute dogs.
I don't know anything about specific fabrics that are better for people with CP. I know that there are such preferences for people with Autism.
I used to work in a home for kids with special needs. That was a long long time ago when I was in high school. They didn't know near as much then and things have changed radically. I did have several favorite kids that had CP. We bonded and were great friends. Basically with the kids I knew, there was absolutely nothing wrong with their brains. Their little bodies were damaged and betrayed them so they could not speak or walk. So sad. The drugs that they were on at that time only made things worse. But my little friends had such a light in their eyes, so I talked to them like I would any other child and they responded.
My daughter has a friend with CP that is an adult now and your would never know she has CP at all. She does have a limp, but other then that she is perfectly normal in every way. My daughter also has a friend with an adopted daughter that also has CP. She came from Russia and when she got here she was really bad. But with therapy and lots of work and love she is now walking and doing really well. It is amazing how far they can go with the right therapy.
I think your friends son would most likely love a quilt made of anything you would make one for any other child.