Go reef, I got an Aussie tusk from DD four months ago. He was a little bigger, 6.5". Like you, I did a long drip acclimation per LA's instructions. When I put him in the DT (I don't QT the fish I get from DD) he went and tucked himself on the sandbed next to some live rock. He stayed there for several days, would swim around once in a while but really barely moved at all and showed no interest in eating. It took several days for him to start swimming around, and a few weeks for him to start eating, and I tried everything. DD actually extended the warranty on him for a full month and it wasn't until the end of that period that he started eating.
Once he did start eating, he got gradually more confident. A little too confident, actually, as he made an expensive breakfast out of my watchman goby - much to the horror of my 10 year old daughter and her friends who witnessed the gruesome act. =:-0. Since then, I've started feeding him frozen krill every day to keep him satiated and there have been no more expensive meals. He's a model citizen even though he's the biggest fish in the tank. He's definitely a showpiece. Gorgeous fish and I was really happy with the way DD stood behind him for a full month.
So, my advice is stick with it, be patient, and he'll come around.