It’s a bit strange that you’re both vegetarian but you ‘secretly’ (I know it’s not a secret but that he’s too young to grasp it yet) feed him meat. It’s good you’re giving him the choice though.
My parents have both been vegetarian for decades and we were never offered any meat at home. Being veggie was not a thing 20/30 years ago in the country I grew up in, so we (kids) knew from a young age what being veggie meant. It was explained in a very matter-of-fact way, that we as a family don’t eat meat or fish. Whenever we were out and about, my parents said they’d be happy if we wanted to try it. But at home, it wasn’t ever cooked. They explained why they didn’t eat meat and their principles for not having it in their home. We were free to try if we wanted but they wouldn’t offer it themselves. It wasn’t ever an issue or required a lengthy explanation. ‘Some people eat it, some people don’t’. Same as some people not having pork for religious reasons.
I think it really was the best approach. I haven’t ever been interested in trying meat (and before someone says that’s because my parents made choices on my behalf - no it’s not. I’ve also never tried Coke and we had bottles of the stuff in the house) - some of my siblings have. I couldn’t give meat to my children because that would go against my principles for not eating it. If I believe a veggie diet is the healthiest/more ethical/etc, that’s what I’ll give my children too, until they’re old enough to decide for themselves. Same with religious beliefs, I wouldn’t ‘hide’ what we believe in or not take them to church until they’re old enough to realise it.
By being raised like this, I realised it all comes down to choices. What we eat/believe in/how we dress/who we support in politics or whatever. No choice is better than the other. Which is why raising your kids according to your choices isn’t any worse than keeping it neutral and giving them the choice later. Keeping it neutral is also a choice, so you’re making a choice on behalf of your child anyway.
I think your son will have to learn at some point that his parents don’t eat meat and why, and that he’s always been eating it. Might be a bit weird to explain but it’s good you’ll support him no matter what he decides.