I like to approach difficult situation with a "two pronged" approach; through faith and through medical. I employ prayer and I turn to my priest regularly on difficult issues. I try to have God near.
Second is medical- I had a traumatic brain injury in '05, roughly 8 months later I began to have night terrors. They happened 5 - 6 nights a week and became rather exhausting. MRI eliminated a tumor and I had no bleeding or bruising that would cause those kinds of disturbances. So, if you have not had him checked by your pediatrician to rule out the most serious, I would. You never want to hear, "gee I wish you would have come in 6 months ago when we could have done something."
Next in the medical is eating. If I go to bed on a full stomach, in other words, if I graze all night long and then go to bed, I almost guarantee myself night terrors.
Final is rest/stress- I saw it was brought up by another poster and I agree. When I get over tired or incredibly stressed, I will have night terrors. Basically, I have had to learn how to pace myself and not over-do things. Not only can I over-do it with my body, but I will tax my brain even quicker. My brain when it is not happy will first interfere with my speech and then if I don't get the hint will give me night terrors. I have learned to do what I should have been doing all along, but now I have a cranky brain that isn't as "forgiving" as it was before I injured it. I'm not saying your child has a brain injury, but they may have a sensitive brain. Sleeping is the time we refresh our bodies and our brains. Dreams are intended for the psyche to deal with things on a subconscious level that we're not dealing with well while we are awake.
Sleep studies are good, just make sure they are board certified. Ask your pediatrician about a referral to a neurologist and ask if, or maybe insist on an MRI or CT scan w/contrast. MRI's are better, and more expensive.
If you ask your child about any potential "issues" try not to ask leading questions. Many times if something sounds good, they will just say "yes" not meaning that it really happened.
One last thing. I know this is a terrible thing to put out there, but I feel it is necessary. Do you have your child in daycare or with a babysitter? Is there a possibility of molestation? If you believe there might be, I would contact authorities and ask for a referral for a forensic psychologist that knows exactly how to approach these things. If, God Forbid, it is true, don't panic in front of your child. That will make them feel responsible. Also, don't rush to them with hugs and saying things like, "oh my God, I am so sorry, you poor baby..." That will only make them feel bad about what happened and feel guilty. The guilt does not belong with a child. Check all your people and make certain they are credible and not into some sort "repressed" memory garbage. He either knows about it and doesn't know what to do, but knows it bothers him or it is not this.
Remember to keep God close. A quick little prayer before you seek information I find helpful, as I believe it will bring me to the people I am supposed to find.