I have been thinking about this and although I ackonwledge fully that it's impossible to categorically know what I would do unless I actually found myself in your (for you at the moment) nightmare of a situation, this is what I think I would do.. and how I would think..
You were keen for another child but DH was not. By a quirk of fate, another child is now conceived. Although you relation is, has been, you say, much stronger since you rebuilt it after his affair, terminating a child that in your heart, you must want very much, will akmost definitely undo all the positives that you have strived to find in your reltionship since you hit, what, a very big blip, and of his making. If I was you I would be very unsure that I could survive with my wellbeing intact, long term, if I did what he alone wanted. If you both wanted a termination it would be different and what had happened in the past (his affair) would be less relevant. But you don't feel the same over this.
I understand you must be very afraid of losing him and this must be back all kinds of past horrors from when you thought he was lost to you in the past. But this time there is a child involved that he is as equally responsible as you are for creating. He is unfairly waivering that responsibilty two-fold; he doesn't wanty a baby not does he want to be involved in the decision to of whether to terminate or not. He expects you to make that decision alone; and to make the "right one".. but for him, not you. He presumaably has given little thought to you state of mind if you go ahead and terminate; the possible guilt, resentment and misery you may well feel as well as initial grief.
If I were in exactly your position, from the info you have given us, and I would tell him I was keeping the baby, give him the opportunity to get his head round that, and support you/continue to make your marriage work. But if he thought he could not do that, then call a day on the marriage. Because you derserve to be in the frame of mind of knowing where you are; whether you are going to be with him, or alone, as you go into a future with your childen. And making that decision yourself, about the baby and the marraige may be the hardtest thing you have ever done, or will ever do, but you removes you from the dreadful powerless position you are in now, whether either option seems dismal. And in the not so distant future, you will be stronger and will cope and I'm sure, be happy.
Wishing you the very best for a positive outcome whatever that may be and thinking of you.