Everyone is going to tell you nightmares, that is what I was told as well. My son did that from the time he was about 6 months to now, hes 16 months. I took him to the doctor and they thought his soft spot was closed, which is not. Thank God for that. But he has a promninant bump on the back of his head, thats been there since birth and they wanted to see what it was. They diagnosed him with Chiari Malformation Type 1. I have never heard of this before and it scared me to death because they were talking about surgery and everything. They did MRI on him and saw that it was not really bad so they decided to wait it out and see if it worsened. He stopped screaming at night because we started giving him Tylenol everynight before bed time. Since we knew it was pain, we could treat it.
I would tell you to take him to the doctor and have him checked for Chiari. Screaming all night is a symptom of it. I will pray that your baby does not have it.
It also could be teething too though. :)