You can feed him some Nori, Brine shrimp w/ siprulina or any herbivore foods. I feed my Yellow tang all of the aforementioned and once in a while put a small leaf of Romaine lettuce on a feeding clip, he seems to like the lettuce more then anything else but he does eat almost everything thrown his way. My Yellow isn't that timid at all but it take 2 or 3 days for him to feel comfortable in his new surroundings. Also make sure all of your parameters are in check as well because if they aren't the fish will become very skittish.
120 Mixed Reef