I can’t agree more than with Scot’s encouragement to make your sessions with him a fun time. Make it a game time, not a workout time.
I might also add that the decision regarding medications is one to be made between the father and the child’s physician. I also suggest that you consult with the child’s physician to make sure that your activity plan (not an exercise plan, even though it is exercise) is appropriate and consistent with the child’s medical history.
I also emphasize Philip’s brief comment about no weights. The epiphysis of the long bones aren’t closed and weight added to body weight can put them at risk.