I'm Dr. CMc and I'll see if I can help provide a bit more information. With what you are describing I would suspect your veterinarian thinks Amango has a form of Lower airway disease. This can definitely be caused by allergens such as pollens, but also can have multiple other causes. Dust, molds, viruses, ect can all be other causes.
It sounds like your veterinarian has tried some of the medications I would generally recommend. With them not making much difference I would highly recommend further diagnostics. These would include a tracheal wash or a B.A.L (Bronchial Alveolar Lavage). These are procedures designed to get a sample of the cells present in the trachea or bronchiols of you horse. Depending on the cells present it can provide information on the cause and help to determine the best course of treatment. Some veterinarians can perform these out in the field, but some are more comfortable with doing them in hospital. Neither are overly difficult to perform and some veterinarians prefer one over the other.
With the fluid sound present, shortness of breathe, and nostril flare, I would definitely recommend getting this done sooner rather than later.
If there is bacteria present, antibiotics may be needed. Depending on the inflammatory cells noted, a steroid like prednisolone may be needed (not used if bacterial infection is present). I have seen some of these cases associated with chronic viral infections requiring antiviral medications.
You could also have thoracic (chest) radiographs done to visualize the lungs. This however needs to be done in hospital as most portable radiograph machines are not strong enough to give good results in the field.
Another possibility could be a pleuritis or fluid around the lungs. As this fluid builds up, it compresses the lungs, making it more difficult to oxygenate the body properly. This could be identified via ultrasound of the chest cavity. Depending on the cause, the fluid may need to be drained, and long term antibiotics may be needed to treat a bacterial cause.
Ultrasound could also identify a potential cardiac (heart) issue that can create similar signs.
As you have not seen the improvement you might expect over the last two months, I definitely feel further diagnostics are in order to help identify the cause and provide the appropriate treatment.
I hope this has helped answer your question. If there is anything else I could help answer, please do not hesitate to ask.