QOS or Static IP will not fix, or even alleviate your problems... they are irrelevant in this scenario. Things like BitTorrent, eMule, or even multi-threaded http downloads do not give a shit whether you have QOS or not.
"Per user/ip/mac bandwidth limit" function is what you need but it is rarely implemented on stock firmware for consumer-grade routers/gateways.
I suggest that you get a DD-WRT/Tomato compatible router from somewhere like eBay and flash the stock firmware with the "Third-Party" firmware.
No offence but be warned you'll need to a little bit geeky to do the above, forget it if your son knows more about wireless routers than you do.