He does need to tell his story in as detailed a manner as possible. Even if he has told it to you in some fashion, ask him to tell it again and while he is telling it ask him questions (what color was it? how long did you stare at it?, etc.) in a very matter of fact manner. I would also raise the possibility of going with him to this cabin (he could take anyone else along too that he chooses) to see if anything happens again.
Kids are growing up today surrounded by a national belief in UFO's and by a constant barrage of TV shows, movies, books, newspaper accounts and religions that treat the existence of aliens among us and alien abductions as fact.
I also have to question your son's motives for both reporting this sighting and for acting (I'm not saying he's faking fear necessarily) frightened. He certainly is getting a lot of attention, isn't he? Is anything else going on in his life that would suggest a desperate need for attention?
Tell him that he can interview any counselor he wants (I wouldn't suggest a psychiatrist at this stage) to see whom he could trust with this information. I would also offer him an opportunity to talk with a university astronomer/scientist about this; I bet you could find a compassionate academic who might give this the right spin without making your son feel stupid.