Buck's Allergies?

So back in December Buck was fed a not-so-good flake of hay....and then promptly started coughing. Unfortunately Buck didn't STOP coughing and I ended up having the vet come out
promptly. That vet told me to give him some Benadryl....well after a week of Benadryl and still
coughing, I called another vet. She came out and listened to his lungs, bagged him (to hear any crackling) and perscribed 8 mL/day of Ventipulmin.

The ventipulmin helped - no more coughing....but once I started discontinuing the cough started up again. We put him BACK on the venti....4 mL/day in the AM. And now we're adding Prednisol - 18 tabs per feeding for 10 days, and then discontinuing it slowly.