As somebody who works in a UK University dealing with such matters (although your son's situation is not one I've exactly come across) I would urge him to speak to the university - his course leader, personal tutor, students union or somebody else he trusts. I'm thinking here about fitness to practise concerns for both fs and your son if the "medical-related" means he is training for a career working with vulnerable people. He must distance himself as far as possible from the drug taking/selling because one whiff of a possible involvement (however unfounded in your son's case) could have serious consequences . His future career might be in jeopardy through no fault of his own. If the police know, or get to know, that illegal drugs are being used/sold in premises where your son lives or has been living, this could appear on future CRB checks. I would urge him to explain his situation to his university asap. They might be able to advise on the housing/accommodation contract issue as well. I know our students union are very experienced in supporting students with housing and contract issues.