Check out Sanrenmu. All their knives would fall well within that budget. SRM 7010 or 7063 good but small. Even smaller is the ZB681. For larger knives Sanrenmu has some but Enlan/Bee brand has more variety. Both are quite good quality and use properly heat-treated 8cr13MoV or similar Chinese steels. Avoid Ganzo: Expensive (relatively) copies and not so good heat-treat. Tend to have very long delivery times however.
If you want just one knife though, I'd suggest the ESEE knives over the Ontario Rat models, or just examine all the new models from Kershaw. They all fall under the $50 range and great knives in their own right although the steel (8cr13mov) remains the same. Kershaw Chill, Strobe, Emerson collabs, etc. I like the Kershaw Skyline and it uses a nicer grade of steel but there are equally nice options out there from almost all companies. Spyderco Delica/Endura can be had around $50 secondhand here in the Exchange.
Browse the For Sale or even For Trade section here with your son, you may be pleasantly surprised.