My wife had some post-partum blues with the first child. She was kind of negative about a lot of things, including me. She'd get upset about things easily, and I did not like seeing my wife upset. Between that and getting up late to feed the baby and everything else going on, I probably wasn't at the happiest point in my life.
It could be he's stressed because of the stress in the household with what you are going through and taking care of a new baby. Part of it could be a kind of empathy. Maybe he feels down because he's sad from seeing you feel down.
I think early in my marriage, my own emotional state was affected a lot by my wife's. If she was happy, as she often was, then I was happy. If she was in a great mood, that made me feel good. But if she felt down, I'd feel bad because she was feeling bad. Later, I realized I've got to stay strong if she feels down and realize that things will get better for her and try to communicate that to her with my attitude and what I say.
I don't know of any instant cures. You can keep your spiritual life up with prayer, fellowship, and the word. Pay attention to nutrition. As you get ready for it, you can start getting some exercise. He can do that. Exercise is supposed to be good if you are feeling down. When you feel up to it, maybe a night on the town or a really fun weekend excursion might help. I don't know if you'd want to take the baby out or even leave her with a parent or in-law at this stage, so the weekend may have to wait. You may be able to leave the kids with someone and go out for dinner or some kind of fun winter event.
) 
