My daughter went through something similar to this when she was younger. She was absolutely terrified at night. She would hear things and see things. Things that were so apparent to us that we didn't even notice, she would not be able to rationalize in her mind and they would scare her so much.
Here are a few things that you should keep in mind.... at 2 1/2 he is not able to really know the difference in real and not real when it comes to some things. For instance, actors in a movie. At his age, he does not understand that they are not really who they are acting or pretending to be in the movie. The same would apply to halloween costumes. Something that he saw could have really terrified him and now he may be dreaming about it. Or maybe something in his room is reminding him of something that he saw that was scary. He is wanting your comfort and your protective arms to reassure him that everything is ok. Maybe you should go to his room with him and ask him if there is something in there that is bothering him or keeping him from sleeping. Then maybe you could explain to him what it is or what a noise is. My daughter is such a light sleeper that when the central heat would come on it would wake her and scare her because she didn't know what the sound was. She would make up in her head that it was something trying to come in her window. I would pray with her and tell her that she has angels that are sleeping with her and keeping her looked after. That seemed to calm her. Maybe you could turn on a fan in his room, that also seemed to help my daughter by drowning out noises. I talked to a dr. about this too. He suggested that I give her Benadryl at bedtime to help her sleep. These are a few of the things that worked for us, I hope they work for you too.