Let me give you some good advise to make things easier on you, until you get him driving his line and understanding momentum put a 67 on the rear with that 12 ( which is still not correct ) and leave gearing alone, Focus on him only pointing things out and preach momentum, momentum, momentum, and tires of course, walk him to the fence while older kids or Sr are racing that are front runners and have it figured out and point out what there doing right, reason being that will end up quickest method to making progress your looking for. I have set down with many little ones drawing the track in the dirt with 2 match box cars showing the line where they should be, and showing where there at and showing them when another kart has position on them and then what to do, bottom line show as much details with the match box cars as you can it really seems to help them with understanding it quicker than just telling them.
Good Luck !!