www.ultimatehandyman.co.uk

Yes ethernet is by far the best. Didn't he get one of the tacwise cable staplers? Great little bit of kit

Wifi extenders/repeaters are utter crap for anything of significance. End of.

Cabled best.
Powerline adaptors good (in most circumstances - don't get the shitty ones)
Wifi - 5ghz good if close to router (less interferance and potential for better throughput)
Wifi - 2.4 if it works well enough

Cable, everytime - there is ALWAYS somewhere to put the cable.

Here's a comparison between my ethernet connection to my router, versus using a wifi dongle connected to the same computer

Wired

D Load 89.68Mbps
U Load 10.08Mbps

WiFi

D Load 20.21Mbps
U Load 4.17Mbps

I selected the same server for both tests
Wifi dongle says it's connected to my computer at a connection speed of 100Mbps too. So I'm sure this shows a true result of the slow down in speed through the wifi interface.

(both tests done using Speedtest.net

JJ - fair point, but in terms of gaming its latency thats the issue (i.e. time it takes in milliseconds for data packet to go from your place to the receving computer) hence why a lot of stock market traders are based within the M25...outwith that the response time is uncompetitive..

If his son is into gaming, I can think of no better gaming machine than a dedicated gaming rig (PC) After all these X Box and PS games are developed on PC's in the first place. A computer with a fast processor and good graphics card, will outperform any of these so called "games machines" any day.

Okay....thanks for all your feedback on this, you've now sold me the idea of forgetting the wifi and going for a cable.
I'm going to hard-wire it into the house though, so I need a socket in the office (bedroom with router in), some cable to run up (in) the wall, through the loft and down into his bedroom, and another socket in his room....is that the easiest way?
How many cores does the cable need? What else do I need to know?

Cat5e cable, solid core Cat 5e 100ft
2 single sockets (one at each end)
Either of the below would work
socket option 1

OR

socket option 2

and one of these to connect the cable to the faceplate (strip outer grey/white sheath, internal wires go on top of each connection, tool "punches" them down and the connector pierces the insulation)

Krone IDC punchdown tool

And a patch cord for each end to connect the socket to the Xbox / router
Patch cords

Most sockets have colour codes on the back - like the following

Awesome - thank you Scot