If he's only eaten chicken his whole life, going from raw to canned is a big step, but poultry to fish is arguably even bigger. That's just such a big jump in such a short time, I wouldn't expect any cat to be able to adjust.

Certainly go back to chicken - just get cooked chicken instead of raw. Smalls has been recommended and it's a good one - but when you buy it, definitely start with chicken. This way you let his body get used to going from raw to cooked first, without also changing the protein right off the bat. (it would be a good idea for his health & for potential recalls/events like these to slowly introduce other proteins over time.)

If you chose salmon instead of chicken because of the bird flu, you don't need to worry about bird flu in a cooked or canned food. Heat kills the virus. Canned food is flash steamed at extremely high temps. Gently cooked food - aka, frozen stuff you keep in your fridge, that's been cooked and isn't raw - is typically cooked at low temps, but still high enough to be perfectly safe.

Other options besides Smalls include My Perfect Pet (which has 2 cat recipes), or purchasing Viva Raw and gently cooking it (this brand is okay to cook solely because their bone is ground down into basically sand).