If you think that is an embarrassing question, here is an extremely embarrassing answer. Did you ever have to 'go' but you weren't near a bathroom? Like in the car, shopping, etc. Then you finally get to the bathroom and nothin?
Maybe it is the same for him... He has to go, you stop him by taking him out and then he doesn't have to anymore? Sorry, that's the only thing I can think of. I don't think there's a way to get him in the mood, maybe a smoke and the morning paper? lol