Well, what I'd do at only 4 months pregnant is make a list of names I liked (ideally no more than 4 names), send it to him via e-mail, and ask him to pick off the list, or if he'd like to add any to the list, to let you know.
Then not mention it again until about 34 weeks, then say again, has he picked off the list yet, if not, you prefer X off the list, although you'd be happy with any of the names. Don't nag, don't discuss it unless he prompts you.
When DS2 arrives, if your DH hasn't discussed it properly with you up to this point, say "hello X." and name the baby youself with your favourite name. If then your DH says he doesn't like it, point out he's had 6 months to discuss it, too late now. If he comes up with something else you prefer to X then fine, use that name. But accept your DH is not going to do what you want, he's not going to get excited going over names and lists, so rather than have 6 months of stress and upset over it, just accept that's who he is and find a way to manage the baby naming process.