In all likelihood he is dying. They can not be spot fed, BTW, in most cases...the graze on various things on the LR so not responding to food isn't particularly a sign good or bad. But they really need mature systems. Even though they may look healthy, damage could have been in transport. As mentioned, they can take up to a month to suddenly die from acclimation shock, and may not show obvious signs of trouble before that. But, sometimes in the right tank (though specific gravity should be higher for these animals, 1.026 or so) they may recover.