He has changed passwords on your JOINT accounts? WHAT??!!
Right, get onto the bank straight away. You need to take action here. He is acting very wrongly and for a start you need to make the bank aware of it - the last thing you need if for him to start clearing out the account etc. Cut to the chase, you can't trust him, he's a complete tool - so start protecting yourself and your baby NOW and don't give him time to rip you off.
What you probably should do: Ring the bank, explain the situation and get an alert put on the joint account so that bills etc. are paid as they should be, but withdrawals are not possible. Ask their advice and say that there is a dispute.
(What I would do: Ring up and say that you can't get into your account and are worried that there has been fraudulent activity. You'll need to prove ID etc. Get a new password set. Then take as much money out of the account as possible and transfer into yours.)
Then get straight onto a solicitor and start the ball rolling. What's the situation with your house? Do you own it? Start packing his stuff. Midwife, GP etc. too. Tell them (and your solicitor) - this is emotional and financial abuse.
If you mean what you say, that you can't come back from this, then you need to let your family know. They will know soon enough anyway! - get on the phone and get some support. You really need to do this. If you want to get him out, easier with someone standing at your side when he arrives back and sees his bags packed.
Oh, he has nowhere to go? Oh no darling, I'm sure your lovely new Facebook friend will put you up :) You are refusing to to go? But darling, I'm afraid I've already reported your emotional and financial abuse of me to the doctor, the midwife, Women's Aid, and my solicitor. Seriously, it would really be better for you if you moved out now rather than harrassed me further :)
I'm not going to bother commenting on his behaviour really - nothing to say, he's a loser, good thing you got the chance to find out now while you're pregnant and not caring for a newborn. Honestly, believe it or not it's a million times easier to do this now rather than after the birth! Get him out, and get onto the bank and get the control of your own finances back - cheeky little scumbag. Oh, and just forget about facebook, adding friends, blah blah - leave him to his little teenage dramas and you've got all the more time to concentrate on getting this lowlife OUT of your hair before your baby is born.
All the above sounds really tough and 100 miles an hour stuff, I'm sure. But... I can tell you, if you honestly think this is a dealbreaker (and I would think so) then if you do this NOW, while he is away and while you do not have a baby to care for, you will have an easier time of it in the long run. Don't take him back. He's shown you who he is - is this someone you want to make a life and a family with? At worst, he's a cheating scum. At best, he is an abusive wanker who's trying to flex his big man muscles and show you who's in charge. Ditch him.