You can take it to Halfords to get it fitted if that's what he's anxious about.
As for the amount of stuff, Men generally can get away with bringing way less stuff than women and children.
Eg., when we go away for a week, my husband can get away with bringing 7 pairs of undies and socks, 4 T-shirts (he's got that thing where he doesn't get as much BO as normal people), his toothbrush, toothpaste and a comb. Maybe a change of trousers/shorts, but if he really wants to travel light he just wears the same trousers all week.
Whereas young children (who are at an active age where they get muddy all the time etc.) will likely need a full change of clothes for every day, plus a couple of spares just in case (unless you want to give up an afternoon of your holiday to going to the laundrette). Plus toys, activities for the journey, maybe a couple of changes of shoes.
And women obviously need more toiletries, make-up, and our clothes generally come in a wider variety of patterns and colours, meaning you can't just wear any top with any skirt. So one skirt might have 3 tops that go with it, and for those tops you need one And it's not exactly terrible to want to look as nice as you normally do on holiday, not to mention preparing for various potential weather eventualities. Unless you're going camping or hiking, or travelling the entire way by public transport then I see no reason why not to bring enough stuff to make yourself comfortable for the entire stay.
Whatever your husband's reasons for not wanting the roof box, it seems he is only thinking about things from his own perspective, and not considering everyone else's needs. Which is incredibly selfish.