I made the same mistake you did of buying him prematurely because I was told by a variety of people that cc stars are hardy.
I've had him for about 1 month and he seems to be doing fine, we'll see I guess.
Anyway, I have mine in cc substrate and I don't think it's bad for them--they climb on rocks it can't be any more uncomfortable.
For future reference, if yours lives and you put him back in the display tank I feed mine cleaner clams from the grocery store as well as scallops. The clams are great because they help stir the sand bed and when the cc star is hungry he gets on top of a clam, pries it open, and feeds to his heart's desire.
Good luck, I hope everything works out.
If someone knows, what are the signs of osmotic shock so I know what to look for. So far, he seems to be doing great.