His mother died 12 weeks ago, on the day his child was born. The death of a parent is horrible enough, but on a day when he should be celebrating the birth of his child? That's a monster blow for anyone to endure. He probably feels racked with guilt for feeling any happiness about his DC because he should be grieving for his mum. One of the happiest moments of his life and one of the worst have fallen on the same day, and I imagine, that's too much to process.
On top of that, you've had a difficult birth, you aren't robust enough to support him, he is not robust enough to support you.
Add on top all the standard pressures that come hand in hand with a newborn.
OP your relationship isn't falling apart, all of that in the space of three months would shake the foundation of pretty much any family unit. No wonder you're feeling at breaking point.
Try and move your thoughts away from feeling that his actions towards you are personal, it's unlikely its you, and more likely its the combination of the issues you are both facing. You're struggling and trying to hold it together and he doesn't know whether he's coming or going and you're sniping at each other.
Phone a counselling helpline, tell them how you feel about your birth. Tell them about MIL, and how he's not coping. They will be much better armed to advise whether you need couples counselling, or grief counselling, post partum counselling, separately, together...
I hope you find the support you need xx