My fiance has Duchenne Muscular Dystrophy, he is 31 and as far as I can gather, he too had this problem so his Mum brought him an electric bed similar to those you get in hospitals ( the bed is called a Super Baltimore ), he is also propped up on pillows under his arms so his shoulders are level across and his elbows slightly elevated, he sleeps on his back with no need to be rolled and just alters his bed to a comfy position, he has no movement what so ever. The same method can be used to prop up the legs I hope this advice helps your son so good luck...