Hmmm...The way the problem is worded makes it sound like the boy's mass is changing!

Anyway, start with a Free-Body Diagram. I've got a weight force (w) acting straight downward, a normal force (N) directly out of the slope, and a resistance force (R) acting up the slope. I'm going to define a coordinate system such that +x is down the slope and +y is in the direction of the normal force.

Since the question deals with an acceleration in the x direction I'm going to do Newton's second in the x direction:

where is the x component of the weight force: where is the angle of the incline. (Make sure you understand why this is the sine function and not cosine.)

So:

You know a and, obviously, g, so we can solve for m:

I get

-Dan