Don't place too much confidence in TalkTalk ! Make sure the router is working OK before you try anything else. Can you use any other internet device on your router ? That is, try another laptop, tablet, smart phone, etc and see if it works. If another device is working OK, then your router would appear to be fine.
Most people connect to their router by WiFi these days, not an ethernet cable. You haven't said what computer your son is using but can he try it with Wifi instead of an ethernet cable ?
Borrow a modern, WiFi-equipped laptop from a friend or neighbour, and see if it can connect or not.
If other computers can connect OK, then it rather points to your sons device. If you have tried another ethernet cable that you know works OK, and it still doesn't work, then it may be the ethernet socket on the laptop. These are easily damaged, especially if they plugged and unplugged a lot of times. They are not really meant to be continually pulled in and out. If the socket is damaged, then a good local PC repairer will be able to fix without problems.
If it isn't equipped with WiFi, then a simple Wifi adapter may be the answer. You just plug this adapter into your laptop's USB socket, set it up the software ( which is easy-peasy !) and you have a wifi adapted laptop !
This will mean that your son won't have to sit near the router, as he can use throughout the house.
I recently bought this little jobby and it works a treat ::::
http://www.amazon.co.uk/Edimax-EW-7811UN-150Mbps-Wireless-Adapter/dp/B003MTTJOY
I hope I have been of some help. Please let us know how you are getting on.