Look at the Americans with Disabilities Act (ADA) site. It has specific dimensions for bathrooms for people who use wheelchairs.

Here is some more information.
Houzz
1.) Go to Houzz Ideabooks. Enter "Universal Design" in the search box. Be sure to use quotation marks.
2.) Go to Houzz.com/Handicap-accessible
3.) Go to Houzz Design Dilemma and enter "handicapped" in the search box.

Universal Design
1.) www.universaldesignstyle.com
2.) www.schaefferconstruction.com specializes in Universal Design

Handicap Access
1.) The Americans with Disabilities Act (ADA) site. It has specific dimensions for wheelchair turning radius, placement of grab bars, everything.
www.ada.gov/adast94.pdf
www.ada.gov/

I'm 19 with CP...my mom's an architect, and a few years ago we redid my bathroom. We tiled the whole floor and most of the walls, and then in the area by my sink, we sloped the floor and added a handheld shower with a bench that pulls down from the wall (no stall). We added a bar to the front of the sink and by the toilet.

Hope this helps!! :) good luck!

I am sure Deborah Butler of Brickwood Builders (look in Houzz Professionals) will have some good ideas for you. :)

@dede lee. Try moving this entire post from "Other" to the "Design Dilemmas" section of Houzz discussions. Many more people and most of the designers will see it there.

Grab bars are only as strong as whatever they're anchored to, so make sure that the contractor puts blocking in the walls (That's a good idea for any bathroom renovation, since most of us will have at least temporary problems with mobility at some time). Another important safety issue is ensuring that the pipes beneath the sink are the plumbing beneath the sink are far enough away from your son's knees to prevent burns, and sheilded. You say the bath is tiny. I often enlarge kitchens (and sometimes baths) by "stealing" space from an adjoining room or closet. Is that possible in your case? Perhaps the existing bathroom could function just house a toilet and sink, to give your son enough room to maneuver, and you can build a shower as close by that as feasible.