I would say this boy is bunking off, or as it is now called, 'school refusal.' It sounds as though his mother is unaware; he may well go to school, be registered, and then leave. A friend's son did this years ago, following an absence for glandular fever; she thought he had returned to school, the school thought he was still ill, and it was only when they contacted her to ask what exams he should be entered for that she discovered what he was doing.
You do have to make sure the mother is aware, but carefully. As you say, the boy is at home, not roaming the streets, the lesser of two evils, but the school will be aware sooner or later.