I don't think this is the big problem. Drinking more water will not make your boxer sick. I think your boxer needs more water. Watch him for a week. If he continues drinking more water get him to the vet.
I would get the blood test to rule out diabetes, just for piece of mind.
It is not uncommon for some dogs to drink tons of water. Keeps has done this a few times, luckily changing his diet to one with more water in it has helped him control his intake. I give my two ice cubes when it seems like they are in the mood to drink too much. That way they still get some water but they are forced to drink it more slowly.