Looking for senior cat food advice

I have a senior cat that when he was about 10 had hyperthyroidism. We got it treated he’s better now but he can’t seem to put weight on. He’s very skinny and is unfortunately a picky eater with a sensitive stomach. I was hoping you guys could suggest some cat foods that can help him put on weight that he’ll enjoy eating and hopefully keep down. Right now he eats mostly canned cat food made from fish but he never finishes his can.