M.L.
I have to agree with Jessica. Men sometimes have so much more on their minds when they know another baby is on the way. Their stress levels go up thinking about whether or not they will be able to provide for another child. I'd take him aside and talk to him. Let him know that his fears are ok and that you will listen to him if he wants to open up to you. Then just give him some space. Guys aren't like us mushy ones where we want to be cuddled with everything.
Good luck and congrats!
M.