Maybe you could try something like this:
Dry Dog Food Sampler Pack - Luke's All Natural Pet Food
Just put several samples in front of him and see if there is one that he seems to really like.
Also, how much does he weigh? Is he a really tiny one? The reason why I ask is that pups with liver problems don't eat well because eating makes them feel sick, so they get picky and eat only very small amounts. They don't eat as much, they don't grow like they should. I would have my pup's bile acid levels tested, just to be safe and rule out liver problems.