Is he a picky eater? If so you could get a letter and request his needs to be accommodated, but they often do it only for medical reasons. I wonder if you could speak to your health visitor to see what they advice.
Otherwise see if they can guarantee to offer him what he usually eats.
My sons nursery don’t allow pack lunches either. But when I spoke with them about how fussy he is and hardly eats anything, the manager sat down with me and went through the whole 3 weeks (that’s their rotation of lunches) of menu and wrote down what he will eat, what can’t be mixes and needs to be separated etc. worth speaking to someone, go through the menu with them and see what happens. Xx
Funnily enough, this happened to me 30+ plus years ago at school! I think that I just ended up getting on with it as a sort of "monkey see, monkey do" type effect. I certainly ate things there that I'd never have eaten at home because others around me were eating it.
I’ll be taking my little boy out at lunch time due to them not being allowed packed lunches and if he gets distracted he won’t finish eating so maybe say to them you’ll be taking him out at lunch times due to him refusing them before x