Special needs stroller
Since your DS can position himself and walk, I would look for a regular stoller with a high weight limit that he feels is comfy.
I have another option, since special needs strollers are very expensive, you might consider a folding wagon. I have a friend who has one, it is made of steel and canvas and can hold up to 150 pounds, and can easily fit in the trunk of your car. If your child can sit up without any problems, this would be a great option (only about $70). If they can not sit up, you probably need a wheelchair type stroller. We have the Ottobock Kimba and love it. I know you can buy a Special tomato stroller which has a very high weight limit (90 lbs), but is about $500.