Mine is very shy and used to be hiding almost all the time in the beginning, he is now a bit scared by the new fish mostly one dottyback blue bar and is seen swimming out of its usual hiding places but it always ate small bits of flakes, at least, sometimes grains and FD artemia. It used to be hidden behind some coral or something then he would dart and grab a flake, it was almost the only time I could see him all day. When he was alone in the tank he used to do this same thing as now, although now he seems a bit more scared by the other fish and tries to pick up the food even faster. Give him some time, although as you mentioned that he was skinny when you bought it, it could already have some trouble eating then. It would be nice to know what food he used to eat as well.