So for many things like this in France, it depends.
There's 'the rule' and then there's what we actually do. If there is a rule (a particular piece of paper is needed, a certain item of clothing is to be worn, a specific rdv is needed for a meeting) then you need to ensure that you can obey that rule. However, in practice, you may never actually be asked to actually follow the rule. You just need to be prepared to do so.
So the rule might be that you need a certain attestation / paperwork to pass through passport control at an airport. But you are never asked to actually produce that attestation, and you pass through countless times without being asked to produce that piece of paper. Unless, of course, one day you go without it (not a big deal, you think - you've never been asked all the other times). On that occasion you will be asked to produce it, and the passport control guys will refuse you entry because you don't have the correct attestation because, clearly, that's the rule and you should have been ready to follow the rule.
The other thing is that there is a high degree of individual liberty on the part of the rule-enforcers. They may insist all the time, some of the time or never. And it can change from day to day, person to person, campsite to campsite.
So long story short, take at least one of each style, start with the ones he prefers to wear and keep the others as back up.
BTW we've lived in France for 20 years, and DH much prefers the tighter swimming trunks, they are much comfier.