I agree with the last two posters. It sounds to me like your son is beyond spoiled. And why did you need to add in there "very big house"? It's great that you have a big house, I'm sure you earned it, but why is that important? and who has a "back up" t-bird? I've heard of back up Hondas, SUV's, Mercedes, Mustangs...but a t-bird?
If my 15 year old son would be so selfish as to say that he wants a Porsche, then steal my 'back up' t-bird, he'd wish he was just grounded until January.