UREA, CREA, and ALKP. <-- Dont know what these are but have you had him tested for diabetes? My uncles lab drinks an insane amount of water, and it turned out to be a result of diabetes.
And I also know another lab that has a cabin near ours who just drinks an insane amount of water for no reason. She's been tested for everything thinkable.
Keely - Yorkie
Haley - German Shepherd
Casey - version 2.0 - Black lab
Jasper - White cat
R.I.P Casey #1.
Gone but never ever forgotten.