If no one has one here, I would recommend you try Ebay. There is always someone selling a stock set off a gun they are parting out. If you get a beat up wood stock you can always shorten it and adjust it to fit your grandson. Or show him how to do it.
Grind down the pad and whatnot. Additionally, it might be cheaper to get a 12ga stock and thin it out a little (if necessary) so it fits the 20ga receiver more flush. A 12ga stock should fit, it will just be proud of the receiver.