there can be many reasons but night terrors would be the most likely. he may be having vivid dreams so real to him that it can frighten him. he may not remember having these dreams but he will wake up (some times) and need comfort. some children can just cry or scream in their sleep without fully waking up. some will get out of bed and come to you. night terrors can last a while and their may be little that can be done to prevent all of these episodes assuming that the does not have any underlying medical reasons for these awakenings. read to him before he goes to bed. read a nice story; a fun story. get his little mind focused on bunnies not scary things. be careful what he sees on television including commercials for movies that can be frightening to him. try to explain dreams to him (it may be difficult at this age) but do what you can to encourage "sweet dreams". a night light may be helpful in his room soft music or white noise (waterfall sounds rain sounds babbling brooks) are also helpful. there are white noise machines that are relatively inexpensive. this may help him to rest better. before he goes to bed to quiet activities. don't chase him around the house and get him all excited. these things will pass in time with love and support. for more information and other suggestions on dealing with "night terrors" an internet search will be very fruitful.
why is my 20 month old waking up during night crying uncontrollably?