With respect, I tell you to forget about all that. It is simply not applicable.
Your son is not going to require XP mode feature of the Windows 7 operating system. That is intended for applications and hardware that are not compatible to operate within Windows 7. You may think of XP mode as a bonus feature add-on, that you are never going to need to use in all likelihood.
I put my pcs to good use and as such, I play many games myself. I've been beta testing 7 for about a year now. I have not run into any compatibility issues and all my games simply work. I do not play too many older games but the ones I've tried are all good. The exception here is the original Wolfenstein.
So I suggest you go ahead with your planned purchase. I don't mind discussing with your son. He's welcome to message me if he would like ask any questions.
If you do purchase, then enjoy the great OS.
Regards.