You're welcome. It seems that there is some conflicting answers you are getting, though... I'm not really sure who is right. On the government's own website, from 2013:
https://www.gov.uk/government/news/housing-benefit-reform-removal-of-the-spare-room-subsidy-fact-sheet
"Parents of students
If a student’s main residency is their parents’ home, then their bedroom will not be considered as spare."
So the trick is to establish that the parents' home is the student's main residency. Apparently, no-one's bothered to define Main residency in actual law, but the main point is that "A person does not necessarily have to be physically present at an address all the time for it to be their main residence." For students, it can be enough to show that:
1. You would be living with your parents if you could, but University work demands that you live away for most of the year;
2. You do intend to return within a year, such as for holidays.
Probably other things as well. How you go about this might vary depending on where you live, since Housing benefit ultimately is administered by the Local Council and some may be more "aggressive" than others about this sort of thing, though that is really speculation on my part. The main point is that if the Council tries to reduce your benefit you will have a good case to appeal against such a decision, so long as your son does indeed return home.
On the other hand, this may well change when/ if Universal Credit is brought in... to answer questions about that it would help to have more information about your personal circumstances (were you live, etc.).