You know what having watched kids. They all have their strengths and weaknesses. So if you son is doodling and having issues with his fine motor skills consider it practice for his motor skills. His handwriting/printing will get better in time. Although I do understand where you are coming from when you aren't certain whether he is doing it just because, trying to get out of work or what. My daughter seems to do this when she is tired of printing or bored by the work. Almost like its an outlet for her frustration. So I let her. Afterall one of the reasons I homeschool is to allow my kids more freedoms then they get in the 'box'
I doodle in church on my notes...I think it helps me to pay attention. I think in some it might be a symptom of boredom but not always. So be careful what you assume is going on.