Oh dear. With a mouth that is stuck open, I would be concerned about a mandible (lower jaw) fracture or dislocation of his TM joint. I would definitely take xray to see if we could confirm one of these conditions. Once we have a diagnosis, we can start a treatment plan (jaw repair or re-placement of the luxated joint).
Convenia is an antibiotic, and both dexamethasone and DepoMedrol are steroids. None of this is definitive treatment for his condition.
Although I don't know how this happened, especially in an indoor cat (fell off a cabinet or something else high off the floor?), I highly doubt that he will improve with a few injections and some massage.
I recommend getting a second opinion locally and requesting xrays. You may even be able to get a referral to a veterinary dentist in your area (there aren't too many of these however).
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