Could be nightmares or nightterrors that are waking him up. The difference between the two is a nightterror is where the child will call for you adn he doesn't reconize you or looks straight through you (it's a very creepy and scary feeling!!). You may be a total stranger to them as well. They are actually still asleep (sleep walking) and you shouldn't try to wake them up but keep them calm and safe. Most of the time it will last under 5 mins and they will go back to sleep and wake up in the morning without a clue as to what happened!! My daughter used to have these a few times a week at first and they slowy fadded away to non at all. She still 'sleep walks' but nothing like the night terrors.
If it's nightmares and he is scared of monsters/booggies man in the closet or under the bed, get him some monster spray. All it is is a water bottle with water in it. And you spray it in the air to keep the monsters away for the night. Talk to him about how they aren't real and make sure he isn't watching too mcuh TV before bed getting his little imagineation going as well.