G.B.
i am a hygienist and my husband is a dentist. we have never heard of an antibiotic staining teeth gray. if a tooth is damaged by a sickness or an antibiotic it is when a tooth is still forming and still under the gum tissue. chances are your son hit his tooth at some point and you just noticed it starting to turn gray at the same time he was having an antibiotic, if it stays gray and you are worried about it you should be able to take him to a general dentist to have an xray to determine if any work is necessary.