Sorry you are in this position OP.
It's a sad thing to say but I'd look at this from the perspective of what's the least worst outcome.
You need to decide this for yourself, but if faced with this choice I'd keep the baby.
The reason being I think the worst outcome would be a living with the emotional impact of an unwanted termination to save (as a result of his ultimatum) a damaged marriage.
The problem here is that you can't put the car back in the bag (or the sperm in the condom more accurately). There's no option that will put you back to where your marriage/family was before you got pregnant.
The only way is forward and I don't think your DH fully understands this. A termination doesn't revolve this issue, it creates a whole new (and very damaging) set of issues.
Obviously the situation would be different if you didn't want the baby, but you do and you're as entitled to your feelings on this as he is (if not more so given whatever the choice it's your body that will be impacted).
In truth, if he felt so opposed to having another child it was his responsibility to stop that from happening after your first child was born by having a vasectomy. He chose not to do this and this pregnancy is the consequence.
If you do decide to have the baby there is a chance he might come around, but regardless I'd be planning to go it alone as I'd find it hard to move on from his attitude to this pregnancy.
Good luck whatever you decide 