Hi Tracey B
I feel for you so much as you face the challenges that come with raising someone in a depression.
From my own experience, having lived with depression in the past (some years ago), I tend to see depression as a deep hole. From the top, we may barely feel it. We may not even notice our self in a depression. Then there are the very depths, which is an incredibly dark and potentially soul destroying place to be. Then, there is all the in between. Sounds like your husband is in the depths of depression.
When someone is at their lowest point, there is no light, there is no obvious way out. It is a truly horrible and torturous place to be. So, the question becomes 'How do I raise them to find the way out?' The tiniest things can make the tiniest differences. Difference is the key. One thing you do not want when depressed is sameness. Tiny steps. Step by step by step means graduating out of the soul destroying sameness and hopelessness. Tiny steps are often about changing our 'I am...'
If you say 'I am someone who never tries anything new' and then you try something new, you become 'I am someone who raises myself to challenges of trying new things'. It might simply be about trying a new food, to start with. This is not too demanding. If you say 'I am someone who is hopeless' and then someone shows you a list they've been keeping of all the new things you've tried, all the challenges you've gradually risen to, you become 'I am someone who has hope when it comes to change, when it comes to making a difference'. Our 'I am...' can be life changing. We change through the tiny steps. To show your husband, in various ways, that he's not the person he thinks he is can be a powerful way of supporting him. It's a way of actively loving someone back to life. It's not enough to simply say 'I love you' or 'You'll be okay', as these are just meaningless words when we're depressed. The love must involve activity. To be loved inactively is depressing. I know this sounds a bit harsh but this is my experience, within a depression.
I know this all sounds a bit romantic perhaps so from the perspective of chemistry or biology, it pays to provide someone with little hits of dopamine to the brain (the 'payoff' chemistry we feel with a sense of achievement). When in a depression, we not initially feel these hits until we eventually recognise them, even in the smallest of ways. This is a start, a part of the graduating process.
Hope this helps a little :)