Advancing Youth

My son is 9 years old and started surfing a couple of months ago. He weighs about 55lbs and has become quite good at popping up on an 8ft soft top. He is able to catch white water and ride it into the beach. However, he is unable to turn, which I am attributing to his size relative to the board he is using. Am I correct? If so, what is the next thing he should be riding? Fish board? Epoxy longboard? I just began myself, so I am really no help to him.
FYI we live in the Baja/San Diego area.
FYI we live in the Baja/San Diego area.