Need help please fiances 22 year old son will not leave?

My fiance has a 22 year old son who works making 300/wk. He will not pay rent, he will not buy his own stuff. He even uses my deoderant which is disgusting. he smokes weed and has to have a drink every night when he gets off work. He was married and decided to leave his wife and move in with us a year ago because he wanted to be closer to this other girl he was seeing. I have caught him with her in our house and he knows she is not allowed in our house as long as he is married. He does not help around the house. Its an argument to even get him to clean his room. I am so stressed out all the time. My fiance wont listen to me he thinks I am being evil when I try to talk to him about it. I feel taken advantage of. I have found inexpensive apts. for him given him the numbers but he wont call he wont move out. He says, "I cant afford it". My fiance is scared that if he says the wrong thing or tries to kick him out he will go running to his mother whom just started trying to come into the kids life 2 months ago. So he walks on eggshells around him. We are supposed to get married next month and I love my fiance dearly but I do not want our marriage to be walking on eggshells around his son. My fiance is suposed to be the man of the house but to me it seems he is putting his son in charge of our home since he is able to do whatever he pleases. I dont know what to do please help. What would you do?