Are you paying? If you are, you need to play the parent card and drag your son into a shop to have him try on boots, choose one, and then be properly fitted. If he won't spend time getting them fitted, you can garuntee that the RS100 will not fit. If he is a good skier I would say that the RS100 is a must (I free ski in the S12 and it is a noodle - I flex it way more than I should and actually regret not going with the race boot). If he is not a good skier, then the 80 should be fine - in terms of flex.
Now, the tricky part about these boots is the last that is inside the last, that cuts down on volume. I skied all last year with that last in, but this season after a few more fit sessions decided that I should take it out and just ski the normal 100mm last versus the 97mm last. There is a huge difference in the boot without the inner last - but I actually prefer it for free skiing. It really needs to be fit, as I have found that the boot has the ability to create a lot of hot spots, as the extra last presents a lot of potential problems that normal boots just dont have. If he is fit properly, he WILL need grinding - or some sort of liner modification.
Make the kid go to a shop. If he won't, don't buy the boots. Just because they look shinny on the shelf or Ski Magazine says that it skis down the mountain for you is no reason to buy a boot. He needs to be fit for his ability, stength, size, and foot shape - not his ideas about asthetic appeal. If you want to spend $600 on a boot buy him a $200 intermediate boot and pay someone $400 to paint it like a Head RS100.
BTW, if you get to a shop and he measures, say a 26.0, and the shop brings out a 26.0 and a 26.5 - and he claims they are too small - don't believe him. I see this with teenage boys all the time. They all want to have the biggest feet and biggest shoes possible. I watched a kid get measured out the other day - he measured just a hair over 26.0. Of course he didn't find a boot that he could actually get his foot into until a 28.0 and that was still too small because he wanted a 29.0. The only boot that would actually fit the kid was a 26.5 - but he refused to put his foot into it because it was too small.
Like I said, pull the parent card and do what you know is right for your kid; not what they think they want or can handle.