Ask him to present a case on how this happened, like he'd need to at work.
He's come up with a half-baked excuse about iTunes cards and you seem to have accepted that, but it doesn't follow all the way through - unless he's got a £600 iTunes card, in which case you could sell that to make up some of your losses.
He's 14, not 4. He knew how much he was spending, and if it occurred over two months, it wasn't one night of madness. He just hoped you wouldn't notice when he got away with it the first time.
Ask him to explain how it happened and what he is going to do to fix the issue. Don't let him involve Apple - if they get involved, that's less for you to worry about, but they might not as he is more than old enough to know better, and you can't teach him to rely on third-parties to fix his errors. Keep the Apple dealings between you and his Dad and let him take responsibility for the whole £600 on his own.
You seem to be letting him hide behind some rubbish excuses when infact, this is a huge issue, hitting the room would have been well justified. React as if he'd stolen your credit card and spent £600 on it, because that's pretty much what he did.