“Contracts” can help @starflame, but you often need a third party, like a counsellor, to assist, because they are not in a relationship with your son, so the dynamics are different.
Basically it means entering into a negotiation, where to stay living under your roof he has to agree to respecting the house rules. These are written out, and consequences for not respecting them are decided on and agreed to by all parties, which means it is discussed, and he gets to say what he thinks is unfair or unreasonable about them, and so do you. The third party is there to help keep things happening politely, using “I” statements and so on, to get the discussion to happen a little formally, but properly.
When an an agreement is reached, everyone signs it, and agrees to review how it’s going, in say 2-3 months time.
The objective is to make your son accountable for his behaviour, and to respect what he is being offered in the home you are providing.
Sometimes this works really well. Relationships Australia might be able to provide free counselling to help this happen.
I hope this is useful for you.