for situations in which the animal is hard to handle, is scared, etc. It is relatively safe. You should be aware that there have been incidents reported with adverse reactions in boxers and sighthounds - you can read about that here":
http://en.wikipedia.org/wiki/AcepromazineHere's an article where they talk about the paralytic capabilities of Ace:
http://www.deesdogs.com/documents/AcepromazineChlorpromazine.pdfIs he a boxer, or a mix with some Boxer?
There was an interesting article some years ago about a study they did back East, or maybe it was a veterinary position paper? Anyway the upshot was that acepromazine has a relaxing effect on the muscles, kind of like paralysis. It does make the animal easier to handle, but doesn't do anything for the anxiety. Somewhat like being awake during surgery - all the scary stuff is there, but you can't really do anything about it. Valium was the drug of choice to deal with anxiety. The problem is that it is very controlled, (DEA) and the vet handing it out to untrained dog owners to be used at the groomer is really asking for liability issues. So you don't see it dispensed much.
Upshot is your dog may have an allergy, it may have been a cardiac episode related to the meds, could have been a reaction to other meds (you mentioned grooming - not supposed to use it while flea dipping). You will not always see the tremors with a seizure, so it may or may not have been one. It is possible that it could have been a cardiac problem related to the Ace. A vet you trust is always a good place to start - but you can do some good reading in Google with searches for "Acepromazine" or "Acepromazine allergy cardiac".
You may want to consult a behaviorist or someone you think is competent and knowledgable with dogs. There are a lot of techniques that can be used to help a dog get used to almost anything. Not saying that is the best for this dog in this situation, but as a general rule you can do a whole lot with little tiny bites of roast beef or chicken at the appropriate times. Not as easy or quick as Ace, but usually you don't have the dog passing out.
And, yes, I have had dogs pass out. The most recent was when an English Setter...got stung by a bee. It appeared much like a severe allergic reaction in a human - panting, swelling of the airway, shaking, drooling - down. Gums still perfusing, heart still beating. I got him up and he kind of rolled his eyes, and by the time we got to the emergency vet he was doing a little better. Nothing like humans getting all excited to stimulate a dog ;)
Hope he's ok!