Studies have shown that anti oxidants can help repair respiratory tissue in horses and other mammals.  You should try adding an anti oxidant supplement such as BioBerry to your rations

I hope the old boy gets over his coughing fits!

HORSLYX LICK
These licks are manufactured using only natural ingredients and no added artificial colours, binders or fillers are used. Carefully formulated to supply essential vitamins, minerals and trace elements. Horslyx Respiratory with menthol, eucalyptus and aniseed and Garlic. I give this to my horse as she coughs quite alot on and of.

Hope it helps :)