Yes Krill should only be a treat. Its kind of like candy for fish.
One thing to try is soaking the other foods in with the krill over night to get that krill taste all over them. If that does not work, try soaking in garlic, they usually like the smell/taste.
If soaking the food in stuff does not work, try putting some prawn on a feeding stick and actually rubbing it on his mouth. I got to the point with my porc that I had to push it quite hard against his mouth and he eventually got fed up and bite at it. I think it was more out of frustration than wanting to eat but he realized it was good. A few more days of the stick and then he ate no problem.
Is this a new puffer? Have you only had him 7 days now? Do you know what he was fed before you got him?
And also check to be sure his teeth are not over grown and his jaw is working fine. It will be quite noticeable if he has lockjaw.
If none of this works, you may have to tube feed him but in that case, I would say something is really wrong and that is A LOT of work. I had to do that once for a puffer with lock jaw (LFS rescue) and it was a real pain. Eventually the lockjaw got better but it took a while.
Hopefully he will eat for you soon. Just be persistent and really "bug" him with the food until he bites.
Good luck!