I put the 13.5t motor on the dump truck, much better than the 17.5t And the brakes are stupid strong on 5 setting, so i dialed it down to 3. The drag brake was factory set on 2, so i went back to 1 which is off. I really think the 13.5t is the best bet for upgrade over the torque tuned motor, which was quite surprising to me. I am using a 5000 50c 2s lipo, so that may be making the best out of the brushed motor!
Wouldn't it be an option to get a cheap remote with EPA's on the throttle channel?
Then you can use the 13,5 motor, but your son can drive it slowely to.
Plus that if he gets more expierenced and wants to go faster you don't have to buy a new motor?
I heard good story's about the Flysky GT3B remote, they are cheap as beans.