Oh OP I'm so sorry you're going through this.
I was exactly the same, awful PND and a difficult baby. It feels so endless and like there's just no way out.
I have no advice to offer really except reassurance that it does end. A year on and my DD is an angel and the bond we have is amazing. Looking back it felt so impossible at the time but you just need to keep going.
Definitely keep on at the doctors, both for your son and for yourself. Different anti depressants work for different people and it may take a while to find the right one for you. Also when I told the gp how o was feeling I was referred for therapy as a priority and seen within a few weeks so try and push for that too.
What I'm about to say is in no way intended to offend you or blame you but it's something I've realised in the past few months. I think that because of my depression, my DDs crying felt much more overwhelming and frequent than it really was and I also think my mood affected her. I've seen such a change in her since I've become happier, but I don't know if this is a coincidence or whether they can tell that you're not happy. Maybe try smiling at him even if he's wailing. I know it's so hard but just try to be bright and reassuring.
But I'm not saying that to minimise what you're going through now. It's sounds truly awful and if you think there's something wrong with him then you know him best and you should keep trying to get help for him.
Remember, you're feeling this low because you love him so much and you don't know how to help him. You're not a bad mum, you're an amazing caring mum who wants what's best for her little boy and he's where he's supposed to be.
Sorry this is so rambly and jumbled, just wanted to let you know there's a light at the end of the seemingly endless tunnel.