From my limited experience working with children with special needs, I have been able to pick up a bit of knowledge in this area. There are a few things to look for.

Muscle tone. A "high" tone means the muscles are always flexed. For example, if you are sitting straight up with your shoulders right back, and then move your shoulders forward and hunch forward.... you have gone from being flexed to loose. If a child has difficulty going from one to the other, they are likely to have difficulty changing positions.

Flipping vs. rolling. When the child is on his back, does he purposely roll over, or... does he end up flipping over because his muscle tone is high? If you lie down on the floor, what muscles do you use to roll over? Pay close attention to what you are doing and in what order. When you observe the child, is he doing the same things?

Muscle strength. All of these types of movements require strength. We have to push our limits a little bit because muscle is "built" when we damage the muscle and it repairs itself. Does he have heavy boots, yet? When he's wearing them, his feet will be heavier and maybe it will be easier for you to hold him up while his feet remain on the ground/floor.

Hope some of that will help. Good luck!