If your son is new to R/C, I would go with a 27 turn stock motor and 6 cell battery.
Wes the 7 cell and the kit motor will be faster, but not the best combination for a new person to R/C. Let him get used to everything and THEN bump up the speed. Also get yourdelf a rnge of pinions so you can dial in the truck to each track.
The 7 cell, stock kit morot, and stock kit gearing is probably what did it in.
Rember "SLOW IS FAST"
Bob Oaks