Had Phyton suggested

[fBechmann]

Getting a compiler environment running should be a minor task compared 
with the complexity of programming in a modern 3D gaming engine, so that 
should not be your biggest fear. And if you choose a 3D gaming engine 
with a decent community there are tutorials how to get everything running.

I would start by thinking about what you/your son want to do:
1) starting 3D game development from scratch, e.g. based on OpenGL
2) using an existing 3D gaming engine and code your own stuff
3) using a complete gaming engine (like quake or UT) and doing some 
mod(ification) development
4) joining the development team of a complete open source 3D game.

Then - for your selection - start seaching the net - if you did not 
choose option 1) the result will quite probably answer your question, 
because each existing "environment" brings its language/compiler with 
it. Too make it more complicated some "environments" provide a 
two-leveled architecture, where the "core" is mostly written in C or C++ 
and the "frame" is written e.g. in python, lua or some other "small" 
language (but too my knowledge 3D games usually have - if at all - 
propriatary languages for this like QuakeC, "normal" languages are more 
typical for RPG); in this case you might choose your favorite 
"environment" with respect to your favorite "frame" language.

Googling should bring you some interesting results, next to this good 
places to start are:
-	http://gaming.foundries.sourceforge.net/	(general gaming)
-	http://3d.foundries.sourceforge.net/		(3D gaming)
-	http://www.planetquake.com/code3arena/		(quake 3 coding)