Perhaps shorter walks might be best when he is having difficulties. I personally would try to leave the harness off of him....or make sure the one you use is lightweight. Do you use the step in harnesses? They are great to get on and off for walks!
Often the symptoms just wax and wane and you don't know why. Sometimes you do. I would just make sure he does less when the symptoms increase...and when they do perhaps have a vet listen to his lungs. As I said, pneumonia can occur and unless treated with an antibiotic, gets worse and of course so does the breathing difficulties.
I had a pup that I would sometimes take out to my car and hold him in front of the a/c vent when he was coughing a lot. Sometimes it helped!
