I would also suggest discussing it with the vet. It might be perfectly normal for him, but you want to be sure. One of the boxers I had as a child had kidney disease (very young) and drinking a lot of water was one of the symptoms. The vet had my mom measure the amount of water she (the dog) was drinking for several days so he could determine if it was within the normal range.
Don't get too nervous about it -- just have it checked out. Any illness will be easier to manage if you catch it as early as possible. I will tell you that my parent's current boxer drinks TONS of water and he is perfectly healthy at 9 yrs, so yours might be just fine!