well, as you have said, he does not have them 50% of the time, so can't claim that he does, even a formal arrangement drawn up and sworn won't help because it won't prove he does have them all that time. The problem is (of course) that for the majority of the time, it will be a couple living in a 3 bedroomed house, which is 2 bedrooms more than they need (most of the time)
i think the only option is for him to move into her place and not have the children so often (or squeeze them all in when needed) or rent privately