I agree with the general concesus: sounds like night terros or cutting molars. It's good if you can determine which one. In my expereince, cutting molars may also be accompanied by congestion/runny nose and/or greenish stool. Night terrors are random, unsettling and have no consitent solution.
We have experienced both in our house with our older two children (now 5 & 7). I'm waiting on the night terrors to start with our now 2.5 year old. maybe I'll get lucky.
Anyway, if it is molars, we found chamomille to be a great solution. you can do this in one of 2 forms - 1/2 a cup of Chamomille tea warmed to the temperature of a baby bottle. let him sip it in the evening before brushing his teeth. this will help him sleep better through the night. Or you can just give it to him when he wakes up to go back to sleep. the second form is a homeopathic tablet that will dislove in his mouth. I like the brand Hylands and you would want to purchase a bottle of Chamollia 30x. give him 1 tablet every hour until he calms down and goes back to sleep. If he's a big boy you may need to increase it to 2 tablets.
For night terrors, there's not much to be done. Sometimes the Chamomillia treatment works, sometimes soothing music is the trick, other times there is no solution but to ride it out with your child. In those cases my husband or I (who ever has a later morning start, will remove the child from the room with the other kids and take him or her to the living room couch. We keep the lights off, no tv or anything and just sleep with the child on the couch. This allows us to preseve our bed as ours but also allows us to be present and comfort a child who has no more idea of what is happening to them than we do.
One final thought, with baby #2 on the way, your child could be exhibiting some anxiety about the impending change. I know it's early in your pregnancy but you may need to start talking about what is going on with him. A great book we used was "baby on the way" by Dr. William Sears. If you are already talking with im about it, you may need to back off or shift the conversation a bit. Our oldest girl, then about 18 months old, began her first round of night terrors during my first trimester with our second child. We didn't make the connection for a while because we hadn't mentioned it to her yet. But in retrospect, we realized our anxiety about miscarrage in the 1st trimester was being felt and acted out by her. When we passed the point of fear, relaxed and began telling people, starting with her, the terrors stopped.
Best of luck.
ABOUT US: Married 11 years, 3 children ages 7, 5 and 2.5.