I suspect your girlfriend's son's vehemence is connected with being a teenager.
Though, remember, for many life-long vegetarians it's a moral issue, right up there with abortion for pro-lifers, gun ownership for NRA members, blocking the borders for anti-immigrationists, etc., etc. It's all a matter of perspective.
Look at the bright side; at least he's not posting your home address and phone number on radical vegetarian websites, inviting radical vegetarians to shoot you in your own kitchen. See? It could be so much worse. All he did was hassle you at a BBQ.
I like tofu, but then I'm blessed with an Asian stepmom who knows how to cook it properly.