Unless I've missed it, I notice that no one has asked how the little boy's mother gets on with YOU? Presumably you have him after school for her convenience? If so, how does she treat you when you drop him off or she collects him? If I'm wrong about that, do you have any sort of contact with her, only I get the distinct impression that she's probably bad mouthing you, to him, and possibly even encouraging this behaviour, and it's quite possible that your son is aware of this but doesn't want to stir up a hornets nest for fear of not being allowed to have contact with his son.