Over here in the UK it is the oak trees at this time of year, just as they are bursting into leaf, that can and do cause allergies in some horses - do you have anything over there near your pasture that could be affecting him?
Over the years I have had several big horses with copd or compromised lungs in one way or another. They were all kept in work and most of them competed regularly. They were kept under normal stable conditions but were fed 5 times a day on high fibre cubes plus additions according to their work. They had small haynets of soaked hay at their last feed late night (11pm).
Without going into loads of background detail, I purchased a mare with very advanced copd, she really heaved and also had the tell tale sign of double breathing that donates 'broken wind'. I followed my normal routine for her and gave her to my daughter to ride (both daughter and mare were 16 years old) The mare seemed to improve a little - was a really lively chestnut hot headed mare anyway! So for one reason or another we decided to join a sponsored ride in aid of a children's charity the following year. I called my vet to do a lung check on the mare, not good! On a scale of 1 to 5 (5 being really bad) he put the mare at 4 and a half!!! However, with nearly 11 months of time in front of us we progressed. This sponsored ride was from South London to York - approx 300 miles - following the trail of the notorious higwayman Dick Turpin who was supposed to have ridden his horse Black Bess from London to York to escape the authorities who were after him.
Of course we worked very slowly but the mare got fitter and fitter over time and her breathing improved. By the following summer daughter was riding her at exercise for up to 4 hours a day, trotting for miles to harden her tendons etc., beach rides for the canter work, not knowing what surfaces she was going to be riding on during the future ride.
Well two days before we left for the ride, the vet came once more to listen - her lungs were completely clear - he had to listen 3 times, at rest, at work and at rest again. He was amazed!! He also said that in his opinion the mare was fit enough to tackle any horse race and win! LOL!! But he wasn't far wrong - this mare was fit. And to prove it she covered 30 miles a day for 10 days and wasn't even tired at the end of each day!!
But I had a secret ingredient that had worked it's magic for this mare.............................carrots!! I cannot remember exactly the way they work - did ask a chemist once and he confirmed it - but carrots, when eaten raw, produce a substance called (I think) hydroxy camphor and this opens the airways and the tubes throughout the lungs. Racehorses running under racing rules have to stop eating/are not allowed to have carrots for (again I think) a week before a race due to the fact that carrots increase the lung/breathing capacity. This mare was getting through 8 lbs of carrots a day during the last three months of her training. And it worked as due to a large amount of local sponsorship she raised several thousands of pounds for the childrens charity by her effort.
So if any of you want to help your lungs perform a bit better (or those of your horses) try carrots!!