The same is true for playing mobile phones in class. He may only listen to the main part, or he may listen while playing. Maybe he has mastered it, and it doesn't matter whether he listens or not.
Actually, it's no big deal not to do homework. I always thought that children would learn better if they did less homework. Too much homework consumes energy and time.
You can get 70 points at the end of the math class in senior two, with a full score of 100, which is still acceptable. Some students work hard, which may be the result.
Of course, if he works harder, he should be able to get better grades.