Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry for this concern for Cooper but agree that this isn't an ER type of situation...not at this point anyway.
Reverse sneezing, is a normal reflex to an irritant in the posterior nasal cavity and any nasal, pharyngeal, or sinus irritation can result in a reverse sneeze. There are a number of possible causes for a this symptom in a dog (which range from nasal mites to foreign bodies to polyps) with allergies also on that list. So, it's certainly possible that this might be the cause for this behavior.
I typically do suggest Benadryl (Diphenhydramine) for my patients when they develop this symptom; the dose would be 1 mg per pound of body weight twice a day. In most cases, it seems to do the trick.
However, this drug can cause sedation for many patients who take it so if he's already lethargic (behavior which isn't typically seen when dogs have allergies), then I wouldn't want you to become concerned that he's worsening in any way.
If possible, I'd take his temperature to determine if he's running a fever. Dogs with allergies should have a normal temperature which is between 100 and 102.5. If it's high, then something else may be going on but if he's eating and drinking, then it still should be fine to wait until Tuesday to have him seen.
I hope this helps. Deb