My dh was very pro-circumcision. I wouldn't recommend our method of 'discussion' because it involved way too much drama, yelling, threatening, things we regretted saying, etc.,. It is an emotional issue, or can be.
We did finally come to terms, thankfully. But make sure your dh knows that the American Academy of Pediatrics doesn't recommend circumsion, nor does any other medical oganization.
Also, other than a general feeling of, "well, I'm circed and I'm ok - I never thought of not doing it" does he have any strong feelings one way or the other? His feelings are valid, but not as reasons for removing part of his son's genitals. If not, hopefully he'll see your side of it and not take it personally - which I think a lot of men do.
Two things that helped my dh think about it rationally (instead of emotionally) is that he couldn't come up with one good reason to do it, at least one based on fact or medical evidence. And also, as I pointed out, our son will always have the opportunity to choose to be circumcised if it's something he wanted to do. But it's not ours, so it's not our decision.
Good luck, no one wants to have these issues to think about during pregnancy. I hope it goes well!
"Home is where the heart is, no matter how the heart lives." - PP&M