Hi OP.
I have a severe nut allergy (life-threatening reactions to peanuts, walnuts, brazil nuts, cashews, pistachio) and moderate reactions to all other tree nuts and legumes.
I'm still here, despite going on three school trips to France in the 90s, holidays there most years and living in France for a year.
Typically, French main courses don't tend to use lots of nuts. Even desserts aren't problematic if you stick to the creamy types like creme brulée, rice pudding etc. Fruit tarts are to be avoided (almonds in the base)
You're right that allergies are nowhere near as prevalent there, or as understood, even by some medical professionals.
The most important bit of advice I can give is that the information given to caterers and restaurants is best done in writing rather than spoken, unless the speaker's French is very good.
In France they don't use a catch-all term for nuts. If you look it up in the dictionary you might see 'noix' but this is the word they use just for walnuts. It's best to say 'I am allergic to all types of nut: peanuts, walnuts, almonds, pistache,' reeling off a list of a few types of nut, being sure to include those you know your DC is most allergic to.
You also need to specifically mention 'huile d'arachide' = peanut oil.
In the UK saying 'I am allergic to nuts' tends to be sufficient for waiting staff and caterers to understand and to take care, abroad we need to be much blunter about it, ie 'I have a severe allergy to nuts. If I eat them by mistake I may die'.
In a restaurant I specifically say 'please can you check my menu choice with the chef and ensure that they don't include...'
I'd produce a card, in big clear type, saying
I am severely allergic to all types of nut.
It is essential that I do not eat any nuts, otherwise I might die.
Please can you check that my plate does not contain any of these nuts: (list of nuts)
Thank you for your attention
I'd give your DS the card, the teachers the card and ensure that the caterer at the residential centre gets it. I never had ANY issues with the caterers on school trips. They seemed to keep things quite basic anyway.
On the off-chance that your DC were to have a reaction in France he and the teachers need to be ready to be clear and forceful about what needs to happen. The pompiers who arrived when I called for an ambulance didn't know what an Epipen was (even when I showed them the used pen) and were baffled when I said I was suffering a life threatening reaction, because I didn't look like it.
'I have eaten nuts. I have a severe allergy to nuts. My airway is closing. I need emergency treatment or I may die'. Repeat it at the hospital reception. Repeat it to the doctors. Never assume that a doctor in France knows that an allergic reaction can be very serious even if the patient is quiet.