Depression is usually seen as a product of dealing with something from the past. Sometimes our husbands can go through stages of grief where they are grieving the life they had before kids...you know, when they had a fun, care-free wife that paid attention to them and did fun things with them lol.
Sometimes they have to grieve the lost of that woman too, just like we have to grieve that part of us. The reality is, that woman we used to be won't be coming back, at least for a long time. So we have to grieve that identity and they do as well.
Seeing a therapist can be a huge help 
I also want to point out a few things that can cause symptoms of depression that are not mentally related but cause mental issues. If he is low in Vitamin D and/or Testosterone this can cause major symptoms of depression. Think "Grumpy Old Man"...this is what happens when men get older. They have low Vitamin D and very low Testosterone. They become grumpy and don't want to leave the house...
With Vitamin D the optimal level for mental health is having the level over 50. With Testosterone a male should be as high as possible without side effects (aggressive behavior). I would work to find a credible endocrinologist and have some blood work done. They should monitor the levels as they rise to make sure that male gets to an optimal level and feels like himself again.
And for the record, this is the same for us women! Obviously, we don't need as much Testosterone but it is still important for us to have it at a level that keeps us from feeling grumpy and depressed. Plus, have your Vitamin D check asap. It can make a HUGE difference!