ok... i will give you another option...
I dont know your kid or how popular he is ...
at 5th grade... his hole life importance is.. acceptance from family and friends.
maybe... he just finally got in with the "cool" crowd in his grade... and they think he was to geeky getting good grades.
Now that he is accepted.. and not caring about school work go him there.
He might be just trying to fit in and not know how to balance grades with peer pressure.
that was just a guess..
only way to know.. is to keep talking to him.