
If his problem has got worse on propolin then I would take him back to the vet if he were mine for a chat as to why this could be the case ;-)
Often, pups are better at night and can hold it more as their system slows right down. They are generally not eating and drinking or on the go, producing adrenalin, getting excited etc - I know that your boy is not a "pup" as such but this could be the answer as to why he can go overnight without messing. I would have thought that if it is a medical problem with him, he would still be doing it over night. I could be wrong of course, just trying to think of a logical reason ;-)
The reason I say the above is because my last boy, who is sadly no longer here, had rear end weakness and heart problems, in the last year of his life he became slightly incontinent and wasn't aware that he was dripping and sometimes making small puddles as he stood up, for all of his 10 years he was an exceptionally clean dog but in the last year, he dribbled very regularly and this also happened overnight. Every morning I would have to clean him and his bed up as he had lost control.