You can get a external hard drive which can plug into a spare USB port on the back of the Xbox or as 'weretheweststand' said you can get a xbox hard drive to replay your existing one. But if you get a bigger xbox hard drive you will have to transfer over all of his saved games onto the new one.
Hope this helps
http://www.amazon.co.uk/s/ref=nb_sb...60+har,aps,200&rh=i:aps,k:xbox 360 hard drive
get any of the ones that say they are for the "slim"
A 20gb thumb drive will do, just put it in the USB port.