Have you tried running him with some dogs that will stay hooked??
x2
I have owned many mtn curs that would run a track for hours if needed and would stay bayed many hours but I have also owned a few culls that would give up and come back.
Have you used a shock collar on this dog? If so this could be the problem, one wrong push of the button can ruin a good dog. Some mtn curs are real sensitive and they need very little descipline or it has to be done right.