I use benylin, which many people found strange till teh vet said he uses it on all his horses, and it is better than any horse products.
Yes he is a specialist equine vet
If he is actually coughing then two bottles of beneylin - normally tickly coughs as he occasionally gets a dry cough - he gets 1/3 a bottle a day split into two.
I have also been feeding him Feedmark's clarity which has helped clear him up - think he had a bit of a autumn cold as he is fine now.