Sounds like night terrors to me. My son started having them around 12 or 13 months. There is nothing you can really do, since he is actually asleep when this is happening. Just continue to hold him or rub his belly until it passes.
Sometimes when my son has them and it is lasting a long time, I will turn on the TV or bring him over to an open window so the cool air wakes him up briefly, and then rock him back to sleep for a minute and lay him back down. But it is better for him if you do not wake him up. Another thing that someone mentioned when I posted about this a while ago was to get a good bedtime routine going, and make sure he is getting enough hours of sleep each night.
You mentioned sometimes he can calm himself...with a real night terror he would have no control over it, but maybe you are observing that he is screaming and then he stops screaming and the terror is subsiding.
If he seems to be awake it could be so many other things, it could be that he is actually having a nightmare, it could be teething pain or having some other kind of pain, it could be reflux, ear infection, etc...but obviously there would be other signs of those things, and they would not happen exclusively at the same time every night.