you say -
"His dad has put so much pressure on him to only eat meat"
"He is really good at starving himself."
yikes!
your DC sounds very strong minded, so don't make it an issue or it will become more than about food - if it isn't already.
When he returns from his dad you said:
"for eg he had a massive rant the other day about how much he hated veggie hotdogs - he had just been with his dad."
This could be more about control and, as others have said, controlling the loyalty towards his dad, who I guess he doesn't live with?
(speaking as a veggie with meat eating kids) I would say absolutely do not make a big deal of this.
As others have said - buy cooked chicken, perhaps make pizzas with pepparami on, how about chicken soups (you can buy etc) if you can't stomach real meat, but be low key about it if you can.
He is old enough to understand that you don't like handling meat and old enough too to perhaps talk about a compromise with you - eg meat on two days a week, lovely (!) veggie stuff the others.
But this could all be about him working out his connection with his dad - and eating his 'dad's' food may be a sign of that.