I completely understand your concerns as a parent.  You also need to understand that sharing that initial post on Facebook, and others sharing screenshots of it, created a sense of panic among the community of families at Kelly.  The police are on record as saying your son's claims were without merit.  If you have any kind of proof that his claims are truthful, and are genuinely concerned about the well-being of all students at Kelly, you need to produce that proof.  If you had no proof, you need to recognize that your social media post was irresponsible and akin to shouting "fire" in a crowded theater.  

Whatever happened, you are right to have your son switch schools.  Either he is telling the truth and he isn't safe at Kelly, or he is lying and you broadcasting his lies has shined a humiliating target on him and made what was likely an already difficult middle school experience worse.