Try tins of sardines in water. You need to break it up a bit for him but it's better than tuna or other predator fish. I have one cat that won't touch anything fish at all. I've had a lot of cats over the years and never seen a cat refuse canned tuna or fresh fish.
I assume there is a lot of salt in that? He can't handle that with some medical conditions he has. We have offered a very small amount to him before just to see if he would eat it and he wasn't interested. For a shelter cat he is "picky" but deserves it. He had a hard life, he's allowed to be picky now.
Not a lot of salt but it def should not be a daily thing. Whatever he likes. My orange Pumpkin cat who won't eat fish loves burritos. Being in Tucson we have lots of great Mexican food. Him and his sister got nuts if I get a fast food burger.