It could be that he's a dick, but there might be something else to it too. It takes a lot of willpower to be vegan and sometimes figuring out where to draw the line is very difficult and won't be understood by anyone else.
I get flack, probably am considered disrespectful for not partaking in family birthday cakes and icecream and all that shit. I don't eat things with added sugar, period. My family, isn't anywhere near where I strive to be there, but I don't expect to change them, so I just apply my principles to myself, while encouraging great desert like foods without sugar. Sometimes I'll bring something that works for me, to share with others.
If he's new at this, he might be working his viewpoint into reality, and learning what can and won't work. He's obviously upset that people like you don't feel a thing and see no issue with slaughtering billions of fish in a corporate machine, but is struggling with reconciling that reality with what he knows to be moral.
In this case, it's fish, but try increasing the situation... what if it was cow? Dog? Soylent green? Your dead enemies.. and so on. At least try to imagine how he feels.
I think he's a bit of a dick. No one man can change the world. The best he can do is live the way he knows he can.. though that in part might be by choosing not to eat in the same gathering where animal life that is dearer to him, is slaughtered.
It's tough. But seems if he can't be at a table near animal products, he's just going to have to throw his own parties or opt out of family events.
