I have one also and he is the same he went really skinny he will eat a couple mysis and a couple brine then he is not interested anymore but he will sift sand here and there throughout the day. I try not to feed any flakes because he just sifts them through his gill I don't think he is eating any.
I have had him for 8 months now he is really skinny but has lots of opportunity to eat as much as he wants I don't worry about him
