In my experience, these friends are always those that actually understand my choices better then those who say nothing.
I think it comes down to 2 things a) Deep down, they feel guilty that someone else has the strength to change their diet for something they believe in OR b) They feel the need to bring you down because they feel you regard yourself higher than them because of your morality.
Either way, the issue lays with them and not you - in almost every case (unless you're going around telling everyone else they suck because they eat meat

FYI I am like that with my close friends haha and they still don't give me crap.) The easier way is to shut them down by saying "Well, I'm sorry I believe in something..." "My diet is proven to save lives, and it matters to me, why does it matter so much what I put in my mouth" etc. and he can't really come back with any logical argument.
Long story short, some people are just a-holes for no real reason and letting their ridiculous comments upset you is something you have to try and get over - unfortunately, only because you'll get it a lot!!

Hang in there, if it's really bothering you, bring it up to him!