I was in your situation once - DD was 10 and I wa desperate to have another. I felt I was over-anxious about the one (was always imagining dire things happening to her, the slightest sniffle became something sinister in my mind!) and thought having another would mean I didn't have all my eggs in one basket, so to speak.
DH wasn't really keen but agreed to go ahead on one condition - that this time, he didn't have to do any of the getting up in the night! I agreed to that and DD2 was born two years later (it took a year of trying this time around.)
I think ideally it's very good that children have a sibling, if at all possible. My DDs are now 29 and 18 and I have the comfort of knowing that if/when anything happens to us (as it does, eventually ) they have a sister for support and just to 'be there'.
You could point those things out to DH (the importance of a sibling for 'roots' in time to come, the eggs-in-one-basket notion), maybe coupled to the promise of no-nights, if that's acceptable to you (I know that sounds a bit un-pc these days, but it was a real sticking point for my DH, and actually when it came to it and the baby was here he didn't always stick to it out of his own choice - I think smetimes men, or women come to that, can't imagine loving another as much as the first, but they usually do once it's here.)
Good luck - hope you find some way of agreeing on this.