Lala, if I were you I would avoid verbal communication with him about the divorce and him moving out.
It has the potential to be pretty unpleasant, and certainly it is a cut and dried way of handling things, but I would advise e-mailing him instead of trying to tell him verbally how things are going to be, which leaves him the option of denying conversations took place or denying that you said certain things.
I will then tell H I have spoken to a solicitor who has recommended getting X drawn up when he moves out in January so he needs to start looking,
If you tell him or email him make sure he knows that you are going ahead no matter whether he is out of the house or not or he will delay leaving in hopes of making you retreat. What you have in mind is an open invitation to him to drag his heels and refuse to start looking. So no 'when he moves out in January' -- tell him in your email (which is considered received and read if you get the automatic notification that it was sent) that you are going ahead with divorce.
I suggest you email him and tell him you plan to be at your parents for Christmas eve, Christmas day and Boxing day and if you really feel he should be there for a short visit he needs to make an arrangement with your parents to visit for a short while on Christmas morning (be specific about the general time of day he should ask them about). Then he should contact them but you should make it clear to him that your understanding is that the invitation to stay isn't extended to him.
Your parents need to tell him he can be there from 10 am to noon (for instance; whatever time frame he is least likely to be drunk is what you are aiming at here) -- they will need to be primed to refuse anything more by way of their time and hospitality. If not then you will have to exclude him from Christmas altogether. They will need to keep the booze locked away for the duration. No convivial offer of a beer or glass of something. Would they do this for you? (Better yet, if it is your parents' habit to attend church on Christmas morning, then maybe he could attend with the family and bugger off afterwards without darkening your parents' door? They could make it plain that the invitation is for church only.)
No matter what is arranged, it needs to be made plain to him that the invitation to be there comes from your parents and that his presence is not expected or acceptable as a matter of course. He is only going to be there if invited and at their sufferance. He needs to feel the cold shoulder.
What he does outside of the hour(s) your parents are prepared to give him is up to him and you are not to worry about where he goes or who he ends up with.