This is going to sound really silly to most folk, but you may be willing to try anything before making that final decision to let him go.
CARROTS! It is so long ago now I cannot remember all the correct terminology, but carrots, when 'broken down' during 'eating' contain something called hydroxy-camphor (Ithink!). This is in fact a natural drug that opens the airways of anyone eating them. Years ago a very well know filly owned I think by the then Aga Khan, was disqualified from winning a Classic Race in the UK because she tested for the 'drug' camphor. It was traced to the fact that she was fed carrots, which according to the drug tests had given her added advantage of 'more wind power' (or something like this).
I researched 'carrots' via a chemist friend and learnt that the above is indeed true.
I then tried it on a horse of mine. I had 'purchased'/saved a 'broken winded' mare for my daughter. This mare 'double breathed' all the time. The vet said she had the use of about 1/4 of her lungs, the rest sounded like crinkly sliver paper LOL! A year later my daughter rode this horse on a sponsored ride from London to York (UK) covering 30 to 35 miles per day approx for 8 days! Two days before we left for this ride the vet came again to check the horse, saying we were mad. He listened, and listened some more. Then pronounced her lungs 100%, and asked for my secret LOL!! (Apart from the carrots the horse was fed a complete food, plus a small wet haynet to help her system keep going through the hours between her last feed around midnight and her first around 7am. She was also stabled, but grazed in hand for 20 minutes a day)
This was a 16 hand horse and she was fed approx 6 lbs of carrots per day sometimes more. You would have to be very careful if you tried this with your little chap, as I would not want him choking on the carrots. Grating them would be an option, but very time consuming. Perhaps slicing them very very thinly and cutting them to about 2 and 1/2 inches long would work (dont leave them full length or one end of them can tickle the back of a minis throat while the latter end is being chewed)
This is just something that worked for me and this horse, and also for my old dressage horse who was nowhere near as bad as the above mare, but did suffer from dust allergies.
Good luck.
Anna