Yep, he's reacting the same way that my xh did after I did what he wanted me to do...get a draft separation written up. I don't know what they expect us to do or what they think the legal system will or will not allow...but they really get smacked w/the hand of reality when it comes down to the separation of assets, custody, child/spousal support, etc. It's like they think that everything will continue as it has been and the only different is a piece of paper that states that they are divorced.
I think he is talking to someone at work or a friend and they are feeding him a line of BS. Maybe it's his mother chiming in when she needs to step aside and allow things to play out. Whoever is giving him advice isn't up on the laws. Sure he could force you to sell the home, but you could also refinance on your own and pay him off. That's what I did and I've never regretted that move.
I am going to say that your h is backed against the wall and never thought for one minute you would go through with a divorce. He knows that the last thread in the apron is getting snipped slowly but surely and he won't be able to come and go as he pleases in the home and the one thing...he knows that you will move on and possibly meet someone new.
In a way, I feel sorry for him. He's lived in La La Land for a few years and now...well, that little fantasy world is starting to crumble for him.
Continue as you have been and please try to avoid discussing the legalities of your situation w/him any more. It's creating more anxiety for him and he's going to come out swinging even more so. You don't need to hear it at this time. The place for that nonsense will be in an office w/a mediator and that mediator will be able to enlighten him w/what he can and can't do or get.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.