another option is to stick with the 27t motor, and buy a used traxxas XL-5 ESC. It has a "training mode" which will only use 50% power. This will both slow the truck down, and extend the runtime of the battery alot. Then, when he is ready for more speed, its just a press of a button to give him the full speed the 27t is capable of. And then, when he wants even more speed, you have a ESC that can handle a much lower turn motor!
Ive often seen used xl-5 speed controls for $25-$35 on ebay. The blue ones are also waterproof, which doesnt hurt
