rosiegirl
I am afraid that we do not deal with Housing Benefit on the forum, however, I have done a quick search and it seems likely that their will be a deduction that will be dependant on your son's income. however, the rates appear to vary from one Local Authority to another.
From what I can see most LA's appear to have a page on their website that explains their policy so I would have a look at your LA's to see what it says, failing that you will need to get local face to face advice, do an internet search for "welfare advice" with your postcode, town or county.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems