How much flow do you have? He could be stressed from low flow. Achilles tangs need TONS of flow. Much more than most fish.
Keep lots of nori available, even if the other fish eat it. He'll catch on soon. It took a few days for my Achilles to start eating nori and almost a week for him to eat meaty foods. But now he eats everything just fine. Once he does start eating make sure he eats lots of nori. And a good variety of frozen food will help. But nori should be the main part of his diet.
Also, if you didnt know already, Achilles Tangs are very, very hard to keep. So even though you got a good deal on him you should read as much as you can on keeping him alive. And starting with flow will be the most important. HTH