As far as optimising the engine, spending a little time (and money) on the chassis Dyno my be worth your time.

This will take out a lot of variables that you find on the track and you can play with it until you find the optimum jetting and timing. Be sure to note the temp, humidity and air pressure on the day you test it. Major changes in these 3 variables when you are at the track may require that you make MINOR jet changes when you actually run the car.

Your basically at the "test and tune" stage and from the sounds of it you've made the big improvements already (15.7 to 13.5). At this point I think one of the main things is to let your son work on his driving and get the car really consistant so you know that the difference in time slips are due to any changes you make and not the way the car is driven. At that point start making changes ONE at time and see how the car reacts. If you start doing 2 or 3 between each run you may do an improvement in one area that is cancelled out by a change in another.

Good luck, it sounds like you and your son are having fun.