I don't subscribe the the lockjaw/krill theory. The theory held promise in the beginning, but has since failed to hold water. I've had fish with lockjaw who have never taken krill. If you've been feeding only krill, it's probably related to a diet deficient in certain nutrients, like vitamin C. There's also the problem of thiaminase, which basically removes all the thiamine from the food AND from your fish itself as the food is being digested. The diet really needs to be varied to make sure you hit all the different food items.
My volitans once went off his feed for going on three months. He'd go right up to the food, look to open his mouth perhaps a little and you'd hold your breath thinking he was going to take it.... but then he would lose interest after a bit and leave. What got that one eating was a few mollies. After he took the first one, it was like he was a crazy dog and sucked them all up immediately. The next day, he was back on frozen foods. I've this happen will all sort of different preds on occasion.
Live foods is always a good place to start to get them interested again. I would also dose vitamin C into the water. Do you have a few pictures so we can look him over?
