So is the issue that he agreed to the original agreement mostly because he was not particularly attached to his original last name since he was estranged from his (presumed) father?
Those facts have changed, the name itself has changed, and he is no longer in the same situation so if you were discussing it fresh from the start, his feelings about his name might affect what conclusion he'd come to, or whether he'd agree to "giving up" the right to pass along his name?
Imagine that this was the situation back at the time of the original discussion....if he'd known his real father and real name at that time, and hadn't been as easily able to shrug off passing it along. At that time, were you actually discussing the issue and coming to some mutual conclusion, or were you laying out what was your bottom line expectation and being clear that this was the way it would be--could he get on board or not?
If it's the latter, then it sounds like his feelings about his name weren't going to sway your feelings about what name your future children should receive. If it was the former, then it sounds like you were open to discussion or coming to some conclusion that wasn't exactly the first choice you were proposing.
Maybe revisiting that can help you decide whether it was a definite, a deal-breaker kind of thing (to give children YOUR name, no matter what) or something that just happened to work out because he didn't have a strong attachment to his name.
We had a similar situation, not that I was determined to pass along my name, but that I was open to either name being passed along. I didn't want to hyphenate, I didn't want to "automatically" pass along my husband's name, I didn't want to use either surname as a middle name or second middle name (that could depend on the name choices, but as a rule I was settled on picking one or the other name as a surname, and giving a first & middle name.)
I was open to using both last names for different kids, too. I didn't like the idea of a gender-based split, but going in order and alternating seemed democratic.
In that case, we'd have our daughter (she ended up with his name), then our twin sons. The first born twin would have had my last name, and the second twin would have gotten his father's last name. (How freaky would that have been for people to deal with?! Multiples with different last names!!! Way to emphasize the whole "these are two individual people" value!
My husband was open to all those possibilities, but not really happy about them or equally comfortable with them. He was able to articulate reasons that he was attached to his name and had some feelings about passing it on (having his children have that name), basically bringing to the table the same feelings that I
was expressing. (That I wasn't comfortable automatically leaving out my name, not even considering options, etc.)
The things we considered included a range of feelings, like "feeling funny" at the thought of explaining what we were doing (explaining to his elderly parents, for example.) It was clear that we needed to "get behind" what we were doing, especially if there was going to be ANY discomfort in straying from the "expected." (Wouldn't want him jumping through hoops, so to speak, for no reason or for something we weren't particularly invested in.)
In the end, aesthetics played the biggest role. Our daughter's name is really clear and straightforward, and his simple English name really suited it. It was perfect.
My longer, difficultly-spelled German name wouldn't have fit. (Though I love my German name combined with his name....us having our individual names and being paired is VERY aesthetically pleasing to me. Though my name, Amy, fits really like a glove with his last name and if I'd changed my own name it also would be pretty classic.)
We had the same situation with the boys and their names....they do fit very pleasingly with his last name. I already had decided that what we did with the first child would set the trend, unless there was reason to reconsider (if we went with heavily German names, or something.) So it wasn't that we were considering using my name with the boys, just that what we chose DOES fit well with that same last name (even though we were planning to use the same last name as we'd used with our daughter, no matter what.)
I do know someone who went with the mom's last name for their daughter. (The dad was adopted and not very attached to his surname; it was an easy decision to make.) They weren't definitely planning to have more kids, but they'd agreed that if they did, they could switch out and use the dad's name for the second child. I have lost touch with them and don't know if they had any more kids, or what they did for names, but the plan was to alternate surname based on birth order, not gender.
If you decide that all along, it was going to be about a mutual decision (and not just whether or not he could agree to YOUR decision and knowingly get on board), then you could revisit the idea and just agree to name them alternating based on birth order (not gender.)
Then if that idea just doesn't fly with you two (because triplets with different last names is too confusing or whatever), you'll have a bit more of an answer.