To make it wireless you'd got a few options
~ Either Buy yourself or get from BT a combined adsl wireless router
~ Buy a wireless access point.
~ Create an
Ad-Hoc Wireless network using your PC as the client.
TBH the first option, whilst not the cheapest, is the best.. and most easy to set up.
Wireless access points arent really that much cheaper than wireless routers.
AdHoc wireless is the cheapest option of all.
Personally I'd use a wireless router and connect that via ethernet (wired) to your own PC like you do now, then just get one wireless device for your sons PC.
IMHO its always best to have "the main PC" or at least one PC connected to the router via ethernet... even Ive managed to accidentally lock myself out of a wireless network - as Im sure will have most people at some time or other if you mess about often enough.
So to recap - you need a new wireless router (or get BT to send you a HH), and one wireless adapter for your sons PC (if the PC hasnt already got one installed already).