Dolphins are not fish, though!
But I agree that you have to start with his stated values and beliefs. You said he's pescetarian mainly for environmental reasons. You also said he's a very empathetic person. So, it sounds like there's something to build on.
It also helps to consider the bigger picture which is that the vast majority of people are complete morons about ethics.
Also, people rarely change their minds about big issues like this due to rational arguments alone. Maybe it would help for him to meet other vegans, interact with domestic animals, observe wild animals etc. And of course to eat great vegan food.
Men also have the additional challenge of veganism and empathy being perceived by the unwashed masses as something feminine.