My daughter (now 6) went throught the same thing, probably from about 7 months or so, until 18 months. It wasn't every night, but she would wake up screaming. It seemed like no amount of comforting would help. She was obviuosly asleep. We held her, rocked her, etc. Sometimes it helped, sometimes it didn't. Sometimes just saying her name and that we loved her seemed to break through. Unlike some others, when she did wake up, she did not become more scared or startled, but calmed down. She was and always has been a very deep sleeper. Depending on how long it lasts (not duration, but how old he is) it does become easier once he can tell you what is wrong - that is if he wakes up. Usually kids grow out of these kinds of things. I know it's tough, but I would try not to worry too much unless it gets to a point where he is in danger of hurting himself or others - sleep walking, getting out of bed, etc.
Good luck!