I have a PS3 not XBox, but we have the same option to save credit card information to make purchase extremely easy. I have a 9 year old son and have chosen not to save any CC information. I purchase Playstation network cards at Walmart or I can add cash to my wallet by going online. I did not want my son accidentally purchasing something without my approval. I also monitor what games he plays and he has his stack and I have mine. This is just another case of bad parenting, or learning the hard way as a parent. Why do parents, really adults in general, in this day and age find it necessary to blame someone else for their mistakes. Just man up, accept responsibility and correct your action.
Also, I noticed he bought an Xbox so his son could play Call of Duty, but "He didn't realise it was costing real money". So how old is his son? I'm sure he shouldn't be playing an M rated game. Parents don't pay attention to what there kids are doing these days as long as they are occupied they don't care. My wife is a therapist and works with kids with ADHD , ODD, autism, etc. While autism completely doesn't apply here, but when kids are diagnosed with ADHD and especially ODD, don't let them play super violent videogames they shouldn't be playing in the first place. You want to really ramp up a kid with Oppositional Defiant Disorder, stick him in front of the tv and have him kill people for hours on end. Sorry for my rant.
M$ not at fault, bad parent bad.