How old is your son???? That will be the telling point here.
If your son is older speak to him, he wouldn’t tell Nonna that he pukes afterwards esp if he wants to indulge. If he’s younger maybe tell her that he must love her delicious cooking so much that he maybe overeats and sometimes pukes during the night. every woman in the house probably keeps putting good in front of him.
Maybe suggest that he eats a bit less while there and suggest he be able to bring home some leftovers for the next night so he doesn’t miss out on the things he loves so much.
Maybe she puts on a massive spread of several dishes and he wants a bit of everything, there is possibly several sweets to choose from afterwards as well, I know going to my mamas for Sunday lunch or dinner there’s atleast 3 sweets to choose from & as a child I always had some of each.

Is your ex not there???? Can you not bring it up with him. No matter what you say to any of them you are playing with fire if it comes across even slightly insulting her cooking!

So he gets a day off school every fortnight? ....his plan is working.

Does your ex MIL or the people who live there give your son alcohol? In some European cultures is more acceptable/it's common to give teens alcohol, so maybe that's what's making him sick but he doesn't want to tell you?

Could be a food allergy if she uses an ingredient you never use at home.

This is an old post.

I would call her the morning after he vomits skidding really worried and asking her what he'd eaten while at her house because he's gotten sick. And id do it every time. At the very least it would help you figure out what he's getting sick from

Yes he clearly has an allergy to something he is eating.

Is it because the food is rich, or does he overeat/get told to keep eating past when he wants to stop, or is it food poisoning because it's left out too long, or does he forget to wash his hands there, or does he get anxiety from being there?

How long has this been going on, and how old is your kid that they're not used to this food and it upsets their stomach? It just seems odd to me.

Talk to your son to find out more information so you can see if it really is the food being too rich.

Can he stop going for dinners and do lunch there instead, so he's got the rest of the day to feel better?

If you say to your ex-MIL "Your food is making my son vomit" she's not going to take it well, so try to work with her to come up with some solutions. "He's been feeling unwell after visiting the last few times, can you think of anything? Has anyone else who's visited/been there been ill? Does he leave his food out of the fridge and come back to eat it later?" or something, don't accuse her of feeding him too rich food otherwise she might just tell you that your food is too bland and that's why it's hard for his stomach to adjust or you're making him weak or something.

Er.. yes, why would you want your kid to continue being ill? 😳

I would.

Umm yes!!!!! Without hesitation