You did not say what your son is using now. It is also not clear if you want to upgrade just the chip or the whole computer. If you would provide specific information like CPU, CPU speed, components, amount of memory, motherboard brand and model (if known), or computer brand and model, people here will be able to give you specific assistance.
In general, the most bang for the buck will come from a good graphics card. Any competent processor paired with a top notch graphics card will yield incredible performance.
A high speed processor with a mediocre graphics card will yield mediocre performance. However, even a modest processor, like a celeron 766 or a Duron 750 (much more powerful) paired with a Geforce 2 GTS, Pro, or ultra will have excellent performance for games.