CJ,
I'm guessing you bought a 360.
You didn't specify what size but that's the only Prairie model for 2008 that Kawi made.

Does the clacking noise go way if rev up the engine in neutral?

If yes, then the noise is coming from the CVT and there is too much cleareance between the sliding "shoes" (or weights) and their slots in the drive converter (pulley housing). The "shoes" are there to spread out the drive sheaves using the natural centrufugal force as the converter turns faster. At low rpm, it's moving back and forth too much due to too much clearance.

I'm assuming you have a warranty. If you do, you need to take it back to the dealer and tell them to check/measure the clearance on driver converter "shoes", and change them out to fatter ones. The P/N of the shoes are 49048-10xx. The xx depends on the thickness of the shoes which range from 7.0 mm to 8.0 mm. For example, if what you have is 7.5 mm, then 7.6 or 7.7 mm shoes should be tried out. Too much clearance in shoes making the noise.

The OVER RIDE button on the left handlebar is used when you want to go fast reverse fater than 5 mph which is considered dangerous. So, Kawi put a program in the ECU so that you can't go more than 5 mph in reverse w/o pressing the button. But don't let your son know about the button.