One thought I have is to wonder what he is drawing. As an extreme visual-spatially oriented person, I'm an incessant doodler. I really need to doodle to focus and pay attention to what's being said. I also used visual diagrams and my own practice work to master math in classes.
People draw their own conclusions of what "paying attention" is; I wonder that this teacher isn't a complete mismatch to your son. If someone thinks that my looking at them is paying better attention, they are wrong. People have many non-verbal ticks and such that can visually distract from verbal content. Presuming this is the first time your son has had math issues, I would again see it as a style conflict with the teacher.
