If you prepare salmon for yourself, could you maybe prepare salmon for yourself at a time that he eats something else (say, at breakfast when he's having eggs or cereal or whatever he normally eats for breakfast) and let him taste a bite or two of yours?

This way, if he likes it you can make it next time for both of you, and if he doesn't it won't go to waste and he won't go hungry. Alternatively, make enough for 2 and plan on eating the leftovers yourself at another meal if he doesn't like it.