The Knight Littlehorn in .50 is a good, smaller option. The previous incarnations (for used market) were the Wolverine and the LK-93(I think?). Thing is, he's 11 now, which means he's probably going to be growing at a fairly quick pace soon, and those are not on the cheap end of muzzies (~$360 for the Littlehorn).
One viable option is to buy a cheap, blued inline .50 and hack off the stock. When he's outgrown it, sell it for cheap to the next dad with a pre-teen killing machine. I don't have much experience with hacking plastic stocks, so take this advice with a grain of powder.