I think sometimes people put mental illness in a different category to other illnesses and assume it's something that can just be "got over", so let me reframe this for you.
Assume that instead of PND, you had cancer. It took you four difficult years to get into remission, during which your life, your husband's life and your child's life was obviously very difficult.
But you're now on tablets which have halted the cancer and you decided to have a baby.
Luckily you got pregnant, stopped taking the tablets, but the cancer returned. You then, very sadly, miscarried.
You're now at a crossroads. Do you go back on the tablets and halt the cancer progression, or do you try for another baby, stay off the tablets and let the cancer spread?
The cancer may not spread too much, but also it could take over your whole body, and there's no way to know how you will be affected.
But the likelihood is that it will be detrimental to your quality of life, plus that of your DH and your existing child. Do you take that risk?
If you successfully have a second child, you may be able to go back on the tablets and stop the cancer again, and everything will be fine, or the tablets may not work any more and the cancer may take over.
I can see why your DH is hesitant, it's a huge risk and he doesn't want to see you distressed and in pain again. Depression is a destructive illness, to the patient and to those around them.
I'm sorry you find yourself in this situation because there is no easy answer, but I can see why your husband has severe reservations.