I am assuming the elderly man is a Public sector tenant and the property is a local authority or housing association property, if the elderly man has been the tenant of the property for some time he will probably hold a secure tenancy. The danger is that when the son has resided in the property for a period as short as 12 months he can probably succeed to the tenancy if this is the first succession.
If the son is merely a visitor the elderly man can ask him to leave, which he is not likely to do from what you say. If the elderly man will not call the police regarding the alleged assault by the son he may have difficulty, my advice would be to ask the son to leave and if he will not call the police to report the assault and have the son removed.