You don't have zero-deductible glass coverage?
Anyway - modern auto safety glass is two layers of glass sandwiching plastic that reinforces it and keeps it from shattering. If he picked up a rock, the crack will be in the outer layer, if he damaged it from the inside (I have done this while trying to remove stickers on the inside of a window using a heat gun... in the winter. Not my finest moment.), the crack will be on the inside layer.
Easy to check by running a fingertip over the crack and feeling for the edge/lip/grab when you pass over it.