I wouldn't worry about it. Just keep the same schedule and he will learn that he can wait for you since you will be home soon. At 9 weeks old he is just a baby and just doesn't realize that he can wait. He is also very very young and despite what the vet says, not all dogs at 9 weeks can hold it for 5 hours.

I would also try to remove his water. He doesn't need free access all day long. Try to give him water in the morning, take itaway, andthen give it back at 3 pm. He may be drinking water out of boredom.