I'm afraid it's just a phase and some kids take a while to get out of the habit. I would first check the physical things, is he too warm, cold etc, is the room dark, does he have a nightlight...research shows that a light in the room breaks the deep sleep pattern. My son who is 5 cannot stay in bed alnight unless it is pitch black, his light is now on the landing.
Also what happens if you don't go to him, try just leaving him, reasure him by telling him it's ok and see if he can get himself back to sleep but don't go to his room.
hope this works and good luck with the pregnancy.