It sounds like you need to get that mower back to the dealer and let him try to figure it out.
Now that the belt has jumped once, likely one or more of the cords in it are broken and from now on it won't stay on period.
If the dealer can't point to some other part of the mower as the cause of the belt jumping, you may just have to buy a new belt.
Being your son was blowing leaves, you can check all the pulleys for debris wedged in them and under the covers. Also, if the front of the deck is excessively low, it makes the belt ride near the top of the idlers where the belt goes up to the engine. Excessively low meaning the blades are 1/4" or more lower at the front than at the rear.