Yes, you can. Talk to the doctor. See if you can get some practical counselling, too, or do some online.
You've been depressed before, so both you and DH must be aware of what strategies work for you. I feel very much for the shock/horror/terror you experience when your spouse says they're thinking of quitting. But I'm going to ask you try and switch your point of view for a bit.
You know how horrid it is to live with a depressed partner - and, yes, the partner has a duty to bear with it & try to help. He's been doing that for a long time, plus his own life isn't exactly a bundle of laughs atm. I feel sorry for both of you!
Can you try looking at this as a cry for help? That is, address his sadness & frustration. Tell him you know how hard it is for him, and you love everything he does to try & help you along. Tell him you're going to STOP being depressed coz you want to get back the couple you were before, only better.
See if you can get him to go to the doc with you!
While I don't know what you're doing to combat the PND, the standard things like joining a dance class, going running (indoors in this weather, heh) do help. Doing girly stuff helps, too. Yes, you have a small baby & DCs, but you REALLY need to start treating yourself like the precious creature you are.
This will reinforce your own health & personality, and be beneficial whatever happens in your marriage.
Sorry, I would have liked to make this longer but hopefully others will step in