lol you have 60 days after the birth to register the bub...so if you don't have a name by about day 55 I'd be getting cross :D only joking.
It might take him a wee while to get used to the whole 'we're gonna have a baby thing'. It's happening to your body, but to him it's just an idea.
Wait until he sees it on a scan or can see your belly and feel it move. He'll get there in his own time.
Why not buy him a names book and leave it wherever he likes to be...on his desk, in his car or something with a little note saying something along the lines of 'thought you might like to have a browse without me nagging'. Put a pen and notebook with it and he might do it off his own bat....worked for mine anyways.
Good luck
eta DH came up with Darcy's name all by himself.....it will happen.