Hi, we had always agreed on 2 DC though I had always told DP I wasn't sure if I would feel a longing for a third as I am one of 3 myself. I did try really hard to convince myself to stop at 2 DC though, wrote pros and cons lists (admittedly the cons always outweighed the pros!).
A while after I had DC2, I still didn't feel that our family was complete, and broached the subject with DH. He wasn't overly keen at first, and I didn't push him, never got emotional etc about it. It depends a lot on your individual circumstances too I think- ie, we are lucky to be able to easily afford it, have enough space in the house, both parents locally who are very supportive and our 2 DCs are great sleepers and pretty easy kids on the whole. When I pointed all of this out to him, bringing the subject every month or so in a calm, non pressuring way, he readily agreed to TTC no 3. It took almost a year to happen, and he seemed more disappointed than me some months when nothing happened!
I know it's not exactly the same situation as yours, in that you'd like DC2 not DC3, but maybe making a list of pros and pointing them out to your DH would help.,if there are any stumbling blocks, eg struggling to afford another maternity leave, try to think of a solution eg you're willing to cut back on a few luxuries to save up for it before TTC, to show your DH that you have considered these things carefully.
These things are hard, we can't help how we feel!