I see this a lot with the people closest to me. People get frustrated because they feel like they're being put onto the defensive. Veg*anism is something which takes a strong moral standing, and contradicts the values which most people were raised with. Try as they might, people cannot help but take it personally. When somebody else takes the moral high-ground, those surrounding them automatically feel guilty.
If they are unwilling or to ignorant to change their ways, they react with frustration and try to turn you into "the bad guy" to offset their guilty consciences.

Kind of like how as soon as I mention being vegan, I am hit with a volley of "I tried being vegetarian once", "I would go veg, but I can't give up [usually bacon/cheese]" and "LAWL You can't eat my meat then, can you

".
It is simultaneously highly amusing, and somewhat infuriating.
I think you need to sit your boyfriend down and tell him how this is making you feel. If he does not change the way he treats you afterwards, THEN you need to consider giving him the flick.
But that's just my opinion.