Answer
This is just like Check Point #2 on page 142.
The rotational axis is at the fulcrum, at the point where the board contacts the triangular support.
The boy's weight produces a counterclockwise torque, which is balanced by gravity pulling down on the board's CG, on the opposite side of the fulcrum.
If the boy is very heavy relative to the board, he must slide the board to the right so he is closer to the fulcrum, giving a larger lever arm to the gravitational force pulling down on the board.
