Not a veggie, no, but in the process of stopping meat and dairy (but keeping fish & eggs). I empathise with your DS. I suppose I feel similarly, I don't feel good about it when I eat meat or dairy and I've struggled with that for a number of years and tbh have had enough, I don't want it. But that is hard in a thoroughly omnivorous family. I also respect that my DH has absolutely no intention of becoming veggie, ever, but enjoys many a veggie meal, and my 3 year old is a yoghurt/cheese/butter-aholic.
So I think good for you for supporting him, given he's too young to facilitate it all himself, and sorting it with school for lunches. I think it's very off for your DP to be feeding him meat hoping he won't notice.
If I was you I would be tempted to try out and find a few veggie meals that the whole family enjoy so that you reduce the simultaneous meat & veggie cooking to every other day or something.
Also consider whether you/he are comfortable with him having food that the meat is picked out of prior to dishing up? eg a sausage & bean casserole or chicken & chickpea stew/curry could potentially be a solution. If there's two types of protein in the same meal then he can just have the bean casserole/chickpea curry and hassle for you is reduced. I know most adult veggies would be revolted by this, but it might be a compromise for now? Obv I would make sure he was aware.
Or build your own pizzas might be fun and allow meat/veggie options.
Bean & lentil chilli is popular here, you could try something like that for everyone?
I make and freeze bean burgers which I then cook and have and sometimes we all have them and sometimes DH and DD have meat ones, with wedges & salad or somesuch. Can post recipe if you want.