Screw welcome, although sorry you're going through such a difficult time right now. You have a few positive things right away in your favour - you say "partner" so I assume you're not married? That makes things a bit less sticky right there legally. Whatever you do, do NOT let him talk you into putting his name on the birth certificate - it will make things unbelievably more complicated for you! He won't be able to take the baby from you. Is the house a rental? Yours or his? Well done for getting away from him so you can be safe. I agree with bounty that seeing a solicitor will probably be quite helpful. Removing him from the house is doable, but it depends on your set up (rental, etc) and who is on the paperwork. Again, a solicitor can help you there.
Bounty wow! You've done loads already! Well done! That initial move (or having him move out) seems like such a huge hurdle, but I found the longer I put it off, the harder it looked. Once it was done and I had a few days to get used to it, I was thinking "I should have done this sooner." My silly comment of the day? Your new place isn't going to seem like home and look "lived in" until... well... you're living in it. 
I have made myself a goal of doing something once a week (minimum) to make the house "mine" whether it be putting up a new decoration or picture or (in my instance where I haven't moved) changing the furniture around. Doing something to create new memories in the house. New recipe to try, new game for the dcs, anything different from the norm, really. It has made quite a difference for me. But honestly, you'll go there when you're ready.
lavender I know where you're coming from on this one. I do know a couple with this dynamic, except the male isn't her boss. She is in her 20s and he is in his late 60s. We didn't want to openly discourage it, but could definitely foresee some problems with it. A few years later now, he is having health problems (related to age and poor lifestyle), he is very possessive of her (as he is worried she'll meet a man more her age and leave him), he cannot get her pregnant (again due to age and health issues) so she now cannot have children in this relationship. It's frustrating to watch. Have you had an honest discussion with her about the drawbacks of these types of relationships? This is sexual harassment at work as well IMO, as he is using his position and authority over her to influence her. Can his behaviour be reported anywhere? If he is not owner of business, then to HR or actual owner? If he is owner, then is there any type of standards organisation he belongs to? Just tossing ideas out there, in case any helps.
Noregrets how aggravating that he is backing out of agreements. Glad that you're able to call him on it and keep things in check though. What a relief it'll be when it's all sorted!
Bounty had to laugh when I read your comments about the email with the excuses. H is like that "I'm sorry, but..." or "I shouldn't have done that, but..." the minute I hear "but" after any type of apology, I know it's useless. He isn't sorry, he's just doing lip service and justifying his actions. And H will also say things to me that he'd never say to someone else or in front of someone else.
topknob What he's doing is horrible, but typical, I'm afraid. Your 15yo may take it at face value initially, but they must be working it out that he is lying. 15yos are pretty observant IMO and understand the dynamics much better than younger children might. Can you talk to your dcs when he is not present and perhaps discuss the situation with them? Not the details, but go over how they are feeling and such.