Hi,
Programming a game can be a very daunting task, especially for a 15 year old! There's quite a bit of work and design that goes in to it. Before he can even think about programming games he'll need to have a good foundation in math, some physics, and programming. These things can take years of study and experience to acquire. However, there are a couple options that you may want to consider that allow you to make games without knowing any programming, giving you access to already created content and tools.
There is a program called RPG Maker, that allows you to create 2D RPG games. Here is a link:
http://tkool.jp/products/rpgxp/eng/
Also, there is a program called GameMaker that allows for a little more flexibility. There is a download for it here, as well as some tutorials on how to make games with it:
http://www.yoyogames.com/gamemaker
I've never used either of these programs (Except for an early version of RPGMaker many years ago), so I can't give you a recommendation on which might be better, but I think both may be good options for you to look in to in the short term.
In the long term, you should encourage him to study math and try some simple programming tutorials with a language such as Python. Choosing a major in college that involves programming would be a good choice (Software Engineering, Computer Science), and if he knows some programming before then he'll be ahead of the game. Programming a game takes a lot of experience, studying, and dedication, so make sure he does his math homework, and he'll be programming games before you know it :p