S.G.
My son used to have night terrors. He would wake up panic'd and sometimes telling me that something has gone wrong. I could not get through to him or get him to calm down at first. Then I learned that it's an anxiety thing for them and they need to control their breathing. Forget about telling him to relax. It won't work. Get him to lay down with you in his bed. Just tell him to breath in real deep. You will have to say it more than once, but he should do it. When he does it, tell him to breath out now. Real long breaths!! Get him to do this for a few minutes. Do it with him. He will start to yawn and then calm down and fall asleep.
If you think he will understand, in the morning tell him that he woke up. Explain that all he needs to do is breath long deep breaths for you. Good Luck!!
He may be stressed in his life. Do what you can to talk to him and give him one on one to see what is bothering him. Sit with him "every night" and read to him. Do this even when he gets older. Make it a habbit.