Pretty much any small-ish fish are fine, including oily ones like sardines and mackeral. As a general rule, I'd aim for fish that don't greatly exceed that he can consume whole as a meal.
Larger fish, like tuna, are not good to feed - they're too high in mercury content (even people should limit how much tuna they eat - and we're quite a lot bigger). Pacific Northwest salmon is also something to avoid feeding raw.