If you can, go back and try to re-read your own post with as much objectivity as possible -- as if you were reading a stranger's post. Hard to do, but possible if you can step back from your own emotions.
I would see a lot of red flags.
He wants a girl. What if a new baby isn't a girl? Will he then push to try a third time? There is zero guarantee on gender.
You have clinical depression; you have endometriosis; you have markers for an autoimmune condition. Not one, not two, but three major health issues on your back. How willing and able is he to do the raising of this new baby while you cope with PPD again (very possible)? While you undergo treatment for an autoimmune disease, if one flares up? Can HE handle a young child and a new baby, on his own, if your medical issue and possibly PPD render you unable to do it for a time, possibly a long time, after the new baby's birth?
If he ends up being the primary caregiver to two kids and a sick or depressed wife, will that cause resentment on his part? Is he fully aware that another child could create this scenario and is he fully ready to take it all on -- just because he rather fancies having a girl that he might not even get?
Unless you and he have had some very long and very frank talks about all this, and he has factored in your illnesses and come away 100 percent ready to deal and keep the family healthy and together -- I would say a baby is a risk in more ways that one.
You said you missed the first two years of your son's life. It sounds as if you have come up from under the PPD that made those early years toughl and that's great. I guess you need to ask a doctor about the odds, with your history, of losing another two years -- of not one, but two kids' lives -- if you have another child. And that is a question not for the ob/gyn but for your therapist.
I'm not saying it'll never happen, just that in your case this is not a simple "Gee, should we"? question. Your serious PPD before, and your diagnoses right now, would really give me pause. I do wonder if your husband fully realizes that you are dealing with these things, or how much they affect your relationships with your son and with him. Adding another child to the mix solely because he wants a particular gender, or because your ob/gyn says "Hurry up or your fertility window will close" -- well, those don't seem like good enough reasons, when compared to your lifelong health.