ATI Outdoors sells one with a retractable stock that I put on the same gun for my two sons, and they killed four birds collectively with it last season. My younger son who was 6 yrs old at the time killed two of those, because the stock goes down to a 10.75" LOP, so it should easily fit your son.
I found out from another member on here who posted about it, and then I've had several friends go the same route for their kids. You do have to retrofit it very slightly for a flush connection, yet that only takes a few minutes and is extremely easy to do.
I will PM you the link and some photos of what mine looks like.
