Wow! This is rough. Maybe you need to get to the root of the problems he is having. It doesn't sound like being pregnant is the reason he is so unhappy, it just put him over. Maybe it is money, maybe he is not where he thought he would be in his career, maybe he is too heavy. Who knows, but he needs to figure it out and being with him for so long, you need him to figure it out. You have a little person on the way that needs a solid family base.
I would try a couple of things. First try to make his life super easy at home so when he is there he feels no pressure. In other words, make him nice meals, ask him about his day, compliment him, etc., keep it even keel. I have always found that this makes my husband calm down and feel at peace. After a few weeks you may see him open up some more and talk about specifics. I have found that my husband needs time to process and if he feels chaos at home he clams up and get crabby.
Once he starts to open up, maybe talk to him about seeing someone about it. It could be as simple as going to your regular doc. He may just be experiencing a chemical imbalance. If he is open to it, you may even want to see if you can get him to go to some counseling. It seems that people lack a lot of tools to deal with life in general, especially since it is not as easy as we all grew up believing with the American Dream and all.
For you, the biggest thing to remember is you need to keep you cool and stay calm. You have to protect that little baby inside you, it is a big responsibility but such a blessing to have. My mother always reminds me that we can only control how we behave and have no control over others, however, happiness is contagious so push for that, maybe it will help.
Good luck to you. = )
P.S. My husband and I just had baby #4 and he was reacting the same way. Didn't think he could deal with it, couldn't figure out where the money was going to come from, just a wreck. I focused on keeping the pressure off him and today he is much better. I think seeing that beautiful little baby and a peaceful home helped him cope.