This is truly heartbreaking.
First off, talk to him today, like you said you would, but be very clear before you start the conversation what you want to get out of it - what your base line of expectation is. Otherwise he will keep inching for more and more until you are stuck well below what you wanted.
For example - do you want him to move out today. Are you happy for him to stay and find somewhere to move out until the holiday? Next month? Work out what you want and stick to it (so if you tell him you want him to move out today but are actually happy to give him a couple of weeks to find somewhere, then you can negotiate to that point). But set that time limit and stick to it. You have the right to assert a clear boundary.
Once you have come to an agreement, speak to the landlord/landlord’s partner and agree that date for him to come off the tenancy. Then make sure that you are solely named on the bills from that specific point (25% single person council tax discount and all that). Not before then, so he is equally liable (though that means shit if he has no money and you are named as well at the moment - it’s just a procedural thing). Get a good divorce solicitor.
The emotional ties and guilt, that feeling of responsibility is wretched. I fully understand that (people pleaser here too raises hand). Look into what help he can get on the NHS for his drug dependency and support him to seek help if you want but don’t let that get in the way of these arrangements. Don’t let him use that support to wheedle his way in again.
If you can, try and talk to your son with his dad (if your husband isn’t being unreasonable), so that he knows that things are going to change but that you both still love him etc.
Then STICK TO THOSE BOUNDARIES YOU SET. (Sorry for the shouty caps). You have the right to have your emotional needs met. You have the right not to be lied to, cheated or used. There will be many times that he will probably accuse you of being unfair but you have the right not to be with someone you don’t trust. His issues are not yours anymore. He will find somewhere to stay and hopefully, start building something of his life again, if you stick to your boundaries, but you aren’t responsible for making sure he does.
I’m so sorry you’re going through this - it’s a horrible situation that won’t get better unless you start making steps to make it so. All the hugs.