Find out what he is scared of. It can become a habit after a bad dream for him to start doing this, whether afraid really or not. Try monster spray, a spray bottle filled with water to spray under the bed and in the closet. If he cannot explain what he is scared about it is hard to help him and tell him that.
Try glow in the dark stars and planets or even something like a dream catcher (my mother in law gave my son one and he is really cool thinking it takes all his bad dreams). Your son already has the habit and has started waking himself up. Yes it is the time of nightmares but he needs to learn that it is okay and don't let him use it as an excuse.
Read happy stories before bed, do reward chart for staying in bed, only leave the door open a crack too. Tell him if he gets up over and over it will be shut all the way again. He sees he is getting attention from saying he is scared, then he will not stop doing that. They are smart at two! :)
Reassure him once, but not over and over again. I think you know him well enough to see if he is really scared or using it as a crutch to stay up or get attention. Be firm, but compassionate about him just not allowed to get up, period. He senses change is about to happen with a new baby too so could be seeking extra attention. Hang in there, stand tough and united with your husband, do not lay in bed with him to fall back to sleep, be sweet and loving but firm and empower him to be a big boy! :)