Depression is a hard one to deal with, especially between the masculine "I should be able to fix my problems and get over it" and the whole "just think positive!" attitude that's somewhat permeated our culture.
Guys don't like to admit that they need help. One thing to consider would be going to couples counseling. If he's severely depressed, it may be that you are doing the majority of the parenting and household work, I don't know your family's situation. However, if he's unwilling to go to counseling with you, I would press on for YOU: "I need to go talk to someone, because I'm really struggling right now with this situation. You won't get help and you aren't getting better. I know you say this is situational, but when this has gone on for years, it's not situational, and it's affecting my life too. I need some support."
Will this light a fire under him? I don't know. But at this point, you have tried to change what he's doing and it's not working, so I would advise you to get help and support for yourself. This doesn't mean that he's the 'bad guy', just that you don't know what to do, you feel stuck, and you need some support. There's *nothing* to argue with about that.
ETA: While some might not 'believe' in anti-depressants, they do work and help some people tremendously. I want to be clear, too-- I do not think YOU are the problem, but do think that support would help you going forward. I also agree that a physical and having his hormone levels checked is a great idea.