Concern for mercury?
In your concern for mercury contents, this depends on the food company itself. The veterinary line of cat food tests ALL their food for heavy metals, toxins, contamination etc. before shipping out for consumption so it is safe to feed. However you need to do your due diligence and contact the company that produces the food you feed to your cat and ask them what kind of testing to they do.
There have been a few studies for fish-based canned diets causing hyperthyroidism, however I wouldn't rush to throw out all of his food in a panic.
Poultry or Rabbit
If you aim for what a cat would eat "in the wild" go for rabbit-based or poultry-based diets. Transitioning should be slow, mix old food with the new food and gradually increase new food and reduce old food over a span of 7 days.
Unlike dogs, a cat WILL starve itself if he does not like the food - try a few different brands and flavors and see what your cat will like best. If he likes the turkey dry you can try to find the canned version of that brand of dry food.
Keep in mind that your cat may have a preference of chunks and gravy vs paté or prefer to have it warmed up before serving.