Get a spray can of Feliway and isolate him in the smallest room in the house, usually a half bathroom. Spray the crap in the air of the bathroon and close the door. Put Havoc in the room. It'll calm his ass down quickly. It contains an enzyme that only cats can smell, that calms them down. I've used it over and over again. Swear by it.

Have either of them come back from the vet? Alot of times, the "medicinal" smell is on the cat and the other cat will pick it up and go nuts. I usually wipe the cat down with a Downy dryer sheet.