One interesting medication is Lomotil. I know it sounds weird, but it does work in some. If your vet hasn't heard of using it, he can find the information in Kirk's Veterinary Manual. I don't know which edition though and I know it is not in all of them. I have a copy of the information from that manual online here:
Index of /lomotil
I have used Torbutrol and also have alternated it with Hycodan for really bad periods. Prednisone can be used but of course you want to avoid it as much as possible...or at least dose it every other day so the adrenals still function. One of my pups was on Temeril P for years for his allergies. When the newer meds came out we took him off of it and he started having problems with his collapsed trachea ... I had not known he had CT and apparently the small amount of pred in the Temeril P had kept him under control. So, that is another med that can work. Also, for really severe nighttime episodes with one of my pups, I used to have syringes on hand that were pre loaded by my vet with torbutrol and ace pro. Yes, it drugged him up...but his coughing was so bad it was necessary. The ace calmed him and the torb helped the cough. It was great for saving me trips to the EC in the middle of the night.
Discuss it with your vet. Probably the best route is to maybe see if the Lomotil will help. That really is a bit of a long shot...but I have seen it help two pups...mine and one other. Then, I think the Torb would be the best next option. It is a bit stronger than the Hycodan I think.
Of course, keep weight down, stress down and keep in a dry cool environment. I have actually taken pups out to my car and put the a/c on high and put them in front of the vent for relief.
Hope something here can help.