Hi Adriano
to Cable Forum
you would need to get a wireless router & a wireless network card for your son's pc.
Do you already connect via a network card (ethernet) yourself, if not you will be much easier if you did, ordinary network cards are very cheap.
You would connect the modem/set top box to the router with a network cable & connect your PC to the router, again with a network cable. Your son would connect via the wireless link.
Cost varies, but you should be able to get a wireless router & card for around £50 - £60, setup is fairly easy, in most cases a 'wizard' on the installation CD will guide you through it.