Probably he would rather be doing something else. Maybe he is a person who tends to procrastinate, whatever the job. Before you decide he is just fooling around, you need to check the kind of assignments he is bringing home. In this case he should be at a different level of instruction. That is yet a different problem, one that may call for a conference with his teacher.
Some children have a very low threshold of tolerance for written assignments.
They involuntarily freeze up when faced with what seems to them an overwhelming task. The problem can be severe enough to hamper seriously a child's progress in school. My wife told me about three or four children in her third-grade classes years ago who had this problem. They were completely bowled over by routine written assignments. They were so intimidated they would not begin, or would finish only a few words before giving up.