Why is my friend so dumb at school?

Like I mean hes cool and everything but just so dumb/ He's going to be 17 soon and he doesn't know simple multiplication like 7 times 7 or something. He like scores really bad and I want to help him succeed and everything. and he has No idea how to do fractions like he thinks a half plus a half is a third. I know he doesn't have mental disorders but jeez... What should I do?