Yes, mad: the only holes/adjustments needing to be made to the car to enable roof rails/bars have already been made when it was manufactured!
Unless you live in a flat, I wouldn't have thought that storing it would be a problem - could you not just leave it in a back yard or pathway or something (possibly under a tarpaulin or similar)? They're fully waterproof in use, so surely they're weatherproof when not in use too? I wouldn't leave one on all the time, as it will impact your fuel consumption, which is just a waste when the box is empty; but for holidays, they're great.
If he refuses, insist that it's either a bigger car or a trailer. Trailers have many uses, but unless you need to carry a very large and/or heavy load, roof boxes are so much easier to travel with (and take much less room to store) if it's just for extra bags of clothes, sports equipment and general household/holiday stuff.
Incidentally, when he prides himself on packing virtually nothing, does that mean that he also uses virtually nothing on holiday; or is he one of these men people who will take for granted that everything they (and their joint kids) need and want will be there, but at the same time berate you for packing it on their behalf, to ensure that it is there?
Are you expecting him to fit it and he's panicking cos he doesn't know how, so having a huge strop about the whole idea?
I gather that they aren't too hard to fit (we've never needed one ourselves yet, as we only have one DC), but Halfords offer a fitting service for the ones they sell. I know you got yours from FreeCycle, but I'm sure you could find one of those people who just instantly know how to do these things on Facebook or by asking around friends who would do it for you for a tenner - then just take photos so you know where to screw it back on when you next use it.