Drawing doesn't necessarily mean you're not paying attetion; doodling can be a way to keep you mind fairly wakeful during a dead-dull lecture - studies have shown that folks allowed to doodle tend to retain MORE than people who are not allowed.
Anyhow, if it keeps up, maybe you can ask the teacher to query your ds as to some relevant info she think's he's missed. If he usually knows the answer, maybe he could be allowed to keep doodling. I was a serious doodler in class, usually in the back of the room, but always had a correct, and often pithy response for teachers - thinking they'd really really caught me off guard they'd select me for a tough question and I'd nail the answer, because despite the doodling, I really was listening.
