D., you're a very good wife to want to help him and to be so understanding. I have always believed we, as a culture, don't know how to handle any mental issues, be it our own or another person's. If someone has a cast, people help open doors, but you cannot see what is hidden in the mind. If he had a broken arm, he'd go get it set, maybe he just needs a small crutch, just for a while, in the form of an anti-depressant to help him get back to himself and to allow him to enjoy the family he created with you. If you can make it about your baby needing a healthy daddy that might help. We parents will do nearly anything for our kiddos, use whatever you've got and get him that help. It's time for the sun to shine again!
If he refuses, I'd be really crafty about serving him salmon on a regular basis and insisting upon a multivitamin. Some studies have shown that we don't get enough Omega vitamins & they really help with this kind of thing! Add flax to a smoothie, give him fish oil tablets if he won't eat salmon. Just tell him to humor you, you're on a health kick and take them yourself, if that's what it'll take! lol. I've gotten my husband to do all kinds of weird stuff in the name of humoring me, and it works out pretty well for the most part.
And I'd add an evening family walk ("for the baby to get fresh air and time with her daddy") or some physical activity to the evenings & weekend to help him get rid of that stress. It eats you up and leaves you spent, completely spent without any relief, and exercise is the best way to get it out.
Finally I'd add keep one day that is a sacred family day - no one can make plans that day - period! You get to play and have fun together. It'll help all of you keep your bond. I can't say enough good things about what "Family Day" has done for us over the years. We live in an amazing area, there's something for everyone to do all the time. Plan and play. We do breakfast, church, change our clothes then go play. It is excellent and has really helped us be a strong family even though my kids are very far apart in age (currently 19/10/21 mos). And it helps keep our marriage strong (well that and a regular date night!)
Just don't give up on him, he'll come around! Good luck to you and your husband!
D.