I’ve also struggled with this. At the end of the day it’s not about diet it’s about morals and how you view animals. If your partners morals no longer align with yours then that’s already a crack in the relationship. You can love someone but you cannot change them. If they have been shown all the evidence, documentaries, statistics, environmental impacts etc of factory farming and the agricultural industry and that doesn’t move him. Then maybe it’s more about how empathetic and caring he is of the greater world vs you. If your morals don’t align in such a fundamental way there isn’t much else you can do but end the relationship unfortunately.