You need to take time to really think about what you want to do.
You say this pregnancy would mean your ExDP would be happy that things were going at his speed, but it's not going to change your mind about how fast you'd be happier going at, would it ?
I think the pressure would definitely affect your other medical issues, so you need to really take time, on your own, to really consider whether you could cope with the huge changes another child would bring.
Also, I'm wondering how your ExDP would cope with having to settle for 4th place in the new pecking order of the 'new' family ??
As you will put both DC before HIM, and obviously you have to put yourself next, as you're the one caring for the children, so they NEED you every hour of every day, for the next 18yrs or so......................
If he was already upset at coming after your DC (which is exactly right !), how will he feel if he thinks 'your' DC is coming before 'his' DC ??
Because you will be putting your DC's feelings first, to help them transition from 'only' child to having a sibling.
I'd not be telling him about the pregnancy at all until you've decided what you want to do. (and if you decided to terminate, I'd not tell him at all as what he's never known about won't upset him, will it?).