If your sure he isen't eating anything icky on the walk then perhaps its because he drinks so much water on returning? Maybe you could try taking a bottle of water and a bowl along with you and giving him little bits every few minutes so that he doesen't drink such a huge amount at once.
What are you feeding him?
Oh and how often do you walk him? If you walk him everyday then perhaps he will get used to it.
__________________ Indy, Benjamin and Malachi   There is no psychiatrist in the world like a puppy licking your face- Ben Williams |