Sounds like night terrors, my daughter has had them since she was about that age. They will usually start screaming,hitting,flailing etc, they may even have their eyes open but they are still asleep. There isn't much you can do when it is happening other then make sure they are safe( not going to fall out of bed...). Usually it ends as quickly as it began and they drift right back into a normal sleep. Occasionally my daughter will wake up and allow me to comfort her. I have found that this happens most often when my daughter is overtired. Make sure your son isn't skipping naps or getting to bed later then his usual time.