G.,
I feel for you! You are in a very tough position because you can see the likely negative outcomes of his choices and it seems like he can't. Of course you want to do or say anything that will change his mind and get him back on track.
My only advice at this point would be to remember that none of this will be resolved in a single conversation tomorrow, but that this conversation can make or break communication between you in the future. That he called you to meet tells me that when the dust settles, he still wants your input and wants you involved in his life in some way.
So maybe focus tomorrow on communicating that even though you don't agree with his choice to move out, you understand his desire to do so - validating his feelings will help him feel less defensive and more likely to listen to what you have to say. Ask him why he wants to move out and really listen to his answers. If you can understand any part of it, let him know that. Then tell him your concerns in the most gentle, non-judgmental way you possibly can. Let him know you're there for him no matter what, but also that you have limits to what you will support because you care about him.
I know it's hard - do your best to be as non-attacking and as gentle as possible. This doesn't mean agreeing with everything he says, it just means communicating that you're willing to listen and that you respect him, even if you don't agree with him.
Good luck with this situation - I know it will take a lot of time and patience. Again, it sounds to me like he still wants your input and to have a relationship with you. Maybe I am overly optimistic, but that makes me believe that you two will find a way to negotiate this.
Good luck - please let us know what happens!