horse coughing mucus

Question
I got this horse a month ago. I realized that he was coughing a little and spitting out mucus. my vet came, checked his lung, gave him a shot of steroid and gave him antibiotic, just in case because being a new horse in the barn did not want to contaminate the others. Well he spend his day in the field and i don't see him coughing. can't watch every minute but seems that when i bring him back in the barn at night at feeding time, he starts coughing and spitting out mucus. I stripped his stall, have rubber mats and put saw dust which is not dusty. prior it was dirt that was dry and let out lot of dust. i will see tonight if he's doing better. meanwhile i took him on trail and he's not coughing. I understand that he might have allergies. wonder if i should give him different feed or hay. at first, when he was on antibiotics, i soaked his hay. he was not coughing, but then again was it the steroid shot or antibiotic or soaked hay. let me know if you can help me with some advises. thanks

Answer
Sorry about the delay. This question got lost in my inbox. :(

It sounds like he just has a problem with dust. There is an over the counter product called anti-hist that isn't too expensive. You could try it and see if it benefited him. You are doing good with the mats and dust free shavings. It could just be seasonal allergies and may go away. Keep the vet apprised of how he is doing since he has already examined him. He will be your best resource.