M.S.
My son has nightmares too. They started around 2.5 and he still has them occasionally, now at 5.5. I find that the more tired he is the more likely he is to have nightmares. At this point I can pin point the nights he will have a bad dream. If the day was really busy or filled with excitement or he was late going to bed, he will be restless for the first half of the night and wake up shaking, crying and mumbling about things. It has scared me a few times, being woken up out of a dead sleep with a freaked out kid :(
So my best advice is to just be cautious with his sleep schedual. The more tired he is the harder it is going to be for him to rest well.
Good luck. Just snuggle that little guy in close :)