That's much better!
In your Thunderbird access account settings and make a note of all the settings for each email address.
Then set up Thunderbird on your son's computer using the same details - it should be hassle free.
http://products.secureserver.net/email/email_t hunderbird.htm
As a bit of extra information, I use gmail - it acts the same way as Thunderbird, fetching my email from 5 different email addresses - including my blueyonder email. The big advantage for me is it is web-based, so I can access my gmail account from any pc in the world at any time.