Mmmm, I think this is difficult.
We are the other way round, we are, or were, all meat eaters and then DD2, at 8, suddenly announces (loudly, in Tesco) that she cannot, ever, eat an animal again, and if I put it on her plate, she will eat nothing at all. Hubby is an ardent carnivore, and I knew nothing about veggie food.
Well, a year on and she is still a strict vegetarian. I've learnt about gelatine in sweets, colourings made out of insects, and all sorts of things. The rest of us are not veggie but we have become very "veggie supportive" and eat veggie meals at least 6 days out of 7 - it's just easier for me to cook.
Coming from where I do now, I can see that it would be hard for you to have meat in the house. If I ever buy meat, DD2 hates it. Everything has to be cooked and kept separate. It's so much easier not to have it around. And in that I think your way round is the more difficult dilemma.
However, lots of people really slated me for "letting" DD2 become veggie at only 8. Lots of my family said "I'd jolly well make her eat it!" To which I replied that I felt she was old enough to make her own life choices and decisions, and I wasn't going to cause years of resentment by making her do something totally against her will.
To that end, I would explain that you cannot have meat in the house because it goes against your moral code and you can't face it. However, I would say you respect his right to choose meat dishes when you are out, or he is at his dads. That way it won't become a point scoring battle. Also, the more you forbid it, the more I suspect he will want it. If you don't make it an issue, who knows, he may return to being veggie one day in the future.
I hope this has helped.