S.C.
Hi A.,
Best I can suggest is that you find some books at the library about fire/firefighters/fire prevention. He probably saw a story on the news or something without you realizing it and he needs to understand that the fireplace is a "safe" place to have a fire. Have you tried sitting with him on your lap at the other end of the room cuddling with him and asking him to look at the fireplace? Maybe he needs to realize that the fireplace isn't moving or changing at all. If he's seen the movie "Monster House" it may have scared him, too. Take things one step at a time and gradually he will come around as long as you don't get stressed while trying to help him deal with it. If he senses your stress he might mistake it with you fearing it as well. When he was littler how many times did you tell him "no" when he got close to the fireplace? He may be remembering that, too. Having his classmates dad come over in full gear explaining to him the safe features of the fireplace may help, too. (Authority figure other than mom or dad.) Have him tell your son "it is safe to be x feet away from the fireplace. The fireplace itself can't catch fire and it won't catch you on fire either." Maybe a demonstration outside might help, too. Can you catch a brick on fire? etc. Always remember, though, a meltdown is his way of communicating with you when he can't find the words. Tell him to look you in the eye, redirecting his face to you, blocking his line of sight to the fireplace and pointing to the space between your eyes if needed, tell him to calm/slow down and use his words. If you can have him in the room not looking at the fireplace and using his words that is the first step to getting through this ordeal. It will get easier the longer you work at it.
Good luck - S.
PS. When my son was that age he had meltdowns over the garbage men taking the garbage away. You are not alone. :o)