I think you can work this out. There are actually many couples out there with the same situation. The one is vegan while the other is not but they still manage to work out their relationship.
I know it's hard since you've been together for a long time and all those time you thought both of you will be vegetarians forever, but people change and so is their preferences. You should try and accept it although I find it unfair that your boyfriend demands it.
Just talk with one another and understand each other. Let him explain his side and listen then explain yours and make him listen.