Both casein and whey are milk derived proteins. Milk contains aproximately 20% wehyand 80% casein. Whey is absorbed quickly and causes a bigger spike in protein uptake than casein, while casein is absorbed over a longer period (think it might be 6 hours or something depending on dose) and results in an overall higher protein uptake. Assuming the protein powder is consumed just after workout and your son will be consuming a real post workout meal within an hour or two after training I think the general consensus is that whey is superior. Studies also show that real milk or choclolate milk is superior to shakes made up of a protein isolate and glucose, probably because the constituents of milk work synergistically. If meat is available right after workout I would do that and drop all the dairy isolate crap tbh, but otherwise whole milk > whey > casein.