What kinds of food have you offered, and how often?
Naso tangs can be difficult to get eating. At 2 weeks and with him so skinny, I'd be very concerned. I've had good success tearing small pieces of nori off the clip and dispersing them into the water column, or flake food. My naso dislikes meaty or hard foods, so mysis and pellets still go unnoticed (after 2 years).