My daughter was climbing into our bed because of bad dreams so I took the dream catcher I had in my room and put it in her window. I explained to her how dream catchers are believed to work and she was intrigued. She has not complained of bad dreams since then.
She also liked my explanation of the dark - might work for you too...
I told her that the nice thing about the dark is that it helps calm our mind and make us feel cozy, like when she was inside my tummy floating around in the dark. (I was recently pregnant so she was really into pretending that she was inside me, being born, etc.)
Come to think of it, your recent pregnancy might have triggered a little regression in the form of being scared of the dark. I'm sure it's just a phase but I would just reassure him that it's ok to feel scared sometimes - maybe he is feeling the pressure of being the oldest child, having to be brave, being a big boy, etc. - sometimes being allowed to "be a baby" without feeling bad about it will get it out of our systems (even as adults we need this sometimes).
Anyway - good luck! I am married 7 years and have a 4 year old and twin 2 month olds so I can relate to your situation!