You can't force him to sell to the Housing Association and he can't force you to buy him out. The remedy would be to go to the court and it would be the court that would supervise the disposal of the property, either on the open market on in one of the other ways suggested by both of you. The difficulty with that is that a court action would cost several thousand pounds and this would come off your respective financial interests in the house. If you can, agree a value and get this sorted out of court. I hope that helps. Please leave a positive rating so that I am credited for my time.