You say you are a complete novice, but is your son a complete novice ?
If not, make sure you buy the correct XBox version for him.
There are two versions (packages) and it may be better to go for the more expensive one. See the write up about the differences here on the Amazon site.
http://www.amazon.co.uk/Xbox-360-Console-wirel ess-controller/dp/B0007SV72A
If he IS a novice, and just starting in the gaming arena, it may be better (and cheaper) to get a PlayStation 2 as there are loads of second hand games available, whereas there are few s/h games for the XBox 360.
Just a thought.