Get him out of the tank now. Sea Urchins release toxins bad enough to kill everything else in the tank and quickly....ours just died tonight and he looked exactly like what you are saying. One of our clownfish just died and the other is probably not going to make it through the night (I'd be very surprised) and the other fish are looking not so great.... :'( Only fish that are doing ok is the one fish in the tank we didn't like and the bristle worm we have been trying to eliminate for ages... Go figure...
