well i'm a kid so it might help that sometimes kid get scared that their parents will yell at them (mine do way to much that i am actually literally dettached from them) kids never want to dissapoint their parents...
and maybe he is suffering in side from something that could have happened at school
talk to him... dont punish him but respect him even though he should repsect him... if you yell at him it will make in vulnerable

milly