Lori, I was thinking about a post Austin made about core body strength and about my GS9's lack of coordination & strength. GS9 is motivated to do some physical training right now, so I got him a book "Pilates for men" yesterday. He has some base measurements for strength through scouts, we'll give this 6 weeks and see if there's any change.
My GS doesn't have as severe symptoms as your DS, but GS is always the last or next to last picked for team sports. After a couple years of swim lessons, then just playing in the water the last 2 years, he has progressed to being an adequate swimmer-- he feels great about that. I'd like to see that kind of growth on land in GS9, too.
Have you ever thought of Pilates or Yoga for your son, and do it at home?