Are you sure you want to go down the legal route?
Surely a better approach is to try and resolve the situation from a family perspective first - unless you want to bring in legal advicce, which is going to put a serious crimp in your personal relationship, as well as upheaval for all the family.
Try and talk to him, when he is sober of course, and explain that you feel his first responsibility is to you and his family, and it's not fair that you are extending hospitality to his friend at the expense of comfor for your children, and time and effort for you in feeding his guest.
Can he not go and stay at his mate's place at least half the time to spread the burden?
It sounds like he is being very defensive when he feels you are dictating to him - obviously you are not, but perception is everything in these situations.
Try having a chat and reasoning with him first - if that fails, and you do want to pursue a legal course of action, your first call should be to the Housing Association for a chat with them, and some advice on how to proceed.