Here's my take CAAD vs. Six.

The CAAD will, all things considered, make a better racing frame if he's thinking race (which, from his athletic competitiveness it sounds like he will be). Racing brings up the huge issue of replacement, etc. "Race what you can afford to replace". If you could get Six's all day long at that price; I might say put him on a Six, but the CAAD9 is going to be a little more forgiving on crash damage and a little more forgiving on the wallet if he does crash and crack it.

CAADs are VERY nice frames, many people buy them and upgrade the components and swear they ride better (as racers) than most carbon frames at twice the cost.

Plus, the CAAD (believe it or not) I think will have the better resale value if he does outgrow the frame. Used CAADs get scooped up left right and sideways by racers (experienced and beginner alike), while a used, raced, carbon frame can sit for awhile (and bring a lower percentage of purchase price) because they scare people. If you have to put in the ad "laid down a few times in races" (and you will) NO ONE is going to buy the carbon frame from you but they'll still be all over the CAAD if there's no big dents in it.

So, I say buy him the CAAD. As he gets into it (or maybe just to start) get him a nice set of wheels with the difference in $$$. A well equipped CAAD will never be the limiting factor in a race for him (unless, maybe, he starts racing over cobbles in Europe, but at that point someone else is paying for his bikes )