If your child has weakness being addressed by PT I would suspect that he has low proximal muscle tone, often found in kids with SPD. In practice, I find that kids with low tone are often in constant motion - staying still requires too much energy. Low muslce tone means there is difficulty maintaining muscle contraction for joint stability and postural control. It's actually easier to be moving than sitting still. The difficulty sitting still certainly can contribute to interference with IQ or any testing.
(responding with my OT hat on)
