Is he fully awake when these happen? The reason I'm asking is that my friend's daughter used to do this. However, she would look like she was awake but wouldn't really be awake. Her eyes were opening, she would be screaming and crying, but she wasn't registering who or what was going on around her. She was still asleep having a night terror.
If it is a night terror there are some ways to help curb them but I haven't heard of a way to completely eliminate them. Some of the causes can be overtiredness and not having a good diet/not eating right. Check his diet and make sure he is eating a decent diet through the day. Also, be consistent with bedtime. Maybe try moving it back a little earlier so that he is getting enough sleep. It took about 2 weeks but my friend worked on her daughter's diet making it consistent and making sure she was eating at the same time each day. She, also, worked on her sleep schedule. Once they were both adjusted and she had been following them for about a week, they noticed a difference. She no longer had the night terrors every night and when she did have them, they tended to be shorter in length.
Talk to your pediatrician to see if he thinks this could be the problem and ask him for some tips on dealing with them as they are happening. My friend did not touch her daughter during night terrors since her daughter didn't know who she was and it made them worse. She just was there to make sure that her daughter didn't do anything or go anywhere where she could get hurt during one. She would talk to her soothingly and after awhile, it would start to register with her daughter and she would either wake up or fall back to sleep.
Anyhow, I hope this helps and wish you the best of luck. I hope you are able to get a good night's sleep soon.