Coding is only a small part of game development. Most important is that he studies many many games: Poker, Backgammon, sports, . . . those are all very important.
He can create games very fast using only paper, pen, and perhaps scissors. This is the fastest way to iterate—especially if he designs alone. Dice and coins can be used for randomization and probability. The core of any game are its rules. He can then use various tools to create the code and content of the game on the computer.
For modding (your interpretation is correct), many modern video games offer free modding tools with documentation that he can use. These games also tend to have active, and (hopefully) helpful modding communities on various internet forums.
A very good friend of mine is a quite legendary video game director: Tomonobu Itagaki. I learn quite a lot from him. He has a personal
Facebook profile, and a
company profile. If your son wishes to hear some tips directly from a professional (and genius, I can ascertain), he should introduce himself and ask some questions.
