My daughter was very visual and verbal. When she was two, she suddenly had a harder time sleeping, and would ask me to "take the pictures out of her eyes." I realized that, though we didn't watch much TV, the video games she'd seen at a friends house and a few movies here and there were "playing" in her mind when she closed her eyes.
Since adults tend to chill out in front of a TV, we think our kids relax that way too-- but for a child under 4 TV/movies are actually very visually stimulating. I recommend cutting out, or seriously cutting back, on TV/movies, especially before bed. Also monitor what is being watched. A lot of Disney movies have graphic, disturbing images that are not appropriate for children under 4--don't trust them!
If you decide to do this, be prepared for opposition. Be clear and firm--make a clear plan yourself for when and how TV will be watched. Decide yourself that you are the adult and will not change your mind--(amazing how hard it is sometimes to be stronger than our kids!) You may have to give it up to a month before you know if it helps your son to be calmer at night. A good sleep is a HUGE investment in the peace of the home and calm attitudes-- so important for learning new concepts. You are wise to see this as an issue to address... especially with another baby on the way! Blessings to you!