Have Cat Questions? Ask a Cat Vet Online.
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).
Please let me know if you have any further questions. If you are satisfied with my answer, please click on the green Accept button. Positive feedback is always appreciated. Thanks.