have you tried different types of nori sheets? how about feeding the tank some meaty foods to see if he gets involved in the frenzy?

when you say he does eat, what is he eating?

if it's used to eating from the glass walls, try putting clips there with small sheets of nori.. my yellow tang goes nuts over the purple ones by julian sprung.. if he's not used to that, maybe try attaching the nori to live rock rubble and placing it on a rock that it always visits