just be aware that you will find even with 'unlimited' ISP packages, a 'fair use' policy will be in the small print and they will probably penalise your lad if he is into heavy 'gaming' on-line.
I will second what BOAC says, it is very rare to find residential ADSL products that don't have a 'fair use' policy where you will find yourself rate-limited and/or packet-shaped for exceeding it.
I beleive Zen were one of few exceptions to the rule, not sure if they've changed their policies for home users now though.