I agree, Smow. When I've rented, I didn't check every (or any) window. Luckily most places have been ok, but there was one flat which I moved into in summer, but in the winter one room was uninhabitable because of mould.
I don't think you can do much with landlords like your son's. If he is forced to make repairs, he'll get away with doing as little as possible. There will then be more problems that turn up.
Do what you can to get them somewhere else while losing as little of their deposit as possible.