i just understand from M2B post that buying in supermarkets is easy wherever you are but the more difficult part is eating out when you never know what they put inside the dish.
That is certainly true. I would be v cautious in France in particular about eating out. Even before I was lactose intolerant, just getting 'vegetarian' understood was reasonably tricky - and that's with fluent French! It really is a cultural thing.
However, for supermarkets, BB is right, there's not such a massive difference, unless you're quite rural, and even then you'll have access to a hypermarket somewhere regionally. You just need to be able to read labels (lait, sans lait, sans lactose, délactosé, etc). Realistically, most of the food you eat is easily lactose-free if it is minimally processed - pasta, rice, fresh fruit and veg, meats etc. You should easily find soy yoghurts ("soja") and it does look like lactose-free milk is more widely available these days. Failing that you'll be able to get plant-milk subs. But it's only for a short period, so please don't worry about it too too much. Of course, he sadly can't have croissants, pain au choco, brioches and all those yummy things, unless you have success with dosing him with lactase drops.