Cohabitation alone of unrelated persons of the opposite sex, is not, strictly speaking, an objective sin. An underlying relationship based on fornication is the real sin, which would not be present here, based on your brother’s lack of intent.
There are two points though where it could be subjectively sinful, temptation to sin and scandal. The virtue of prudence and the availability of alternatives rule both questions.
If your brother, upon a self-examination taking into account all wisdom and discernment, would feel unwontedly susceptible to sexual sins based on this living arrangement, he must avoid it out of prudence. That doesn’t even necessarily mean an active sexual relationship; any serious sexual sin would be enough consideration to avoid it. On the other hand, if the main alternative instead only means living next door, and those same temptations would exist there too, then this consideration of avoidance may lack a causation element, and thus be an irrelevant inquiry. In short, if he is tempted to the same degree no matter what, then trying to avoid sin is a moot question.
Scandal is also sufficient grounds to avoid the arrangement. If his actions were likely to cause another person to think that this living arrangement was ideal or even non-dangerous, or no problem with the Catholic faith, that would lead to scandal. Fortunately or unfortunately, it is hard to cause scandal these days. The people on campus would not likely be influenced by your brother’s choices one way or the other. If he or you announced the arrangement to younger impressionable friends and relatives, that might lead to scandal, but again, the virtue of prudence steps in to either avoid the subject or to make sure that you go out of your way to correct any potential misunderstandings that may arise. Because of scandal, you should look for alternative arrangements that are reasonable feasible; obviously, those don’t always exist. If they do exist, make sure you don’t lie to yourself.
In conclusion, cohabitation is not a direct sin, but the related dangers of unnecessary temptation and scandal that would traditionally prohibit the situation are diminished, making this possible since the virtue of prudence may not be transgressed. Pray for discernment when you make this analysis and look for alternatives that avoid the question altogether. God bless!