I can't find a definitive guide to this despite several searches.
There appear to be two routes, first, he is declared is not considered as part of your household, this would be the same as you were found to not be partners, Alternatively, you should be able to justify him needing a second bedroom the only specific requirement appears to be his receiving the PIP Daily Living component.
I would have a look at the website for your Local Authority, the most detailed information I found was on LA websites.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems
