I don't know. You are going to have to wait for the results of his tests.
It is possible the Prednisone was giving him the 'boost' to be able to cope with normal exercise, and maybe that was why he was able to walk/run 6 miles with no coughing...? But maybe underneath that, it was masking a problem which hasn't been solved yet.
I hope the results will show up something which is easily treatable, and he will be okay.....