I second the opinion of taking him in for more test. Drinking and pee a lot, plus not gaining weight is a sign of diabetes in people and could be in your dog's case too.
Also, I found out this past Sunday when I got my shih tzu, there is such a thing as pants for dogs. Apparently you put a pad in them so they don't mess in the house.
Good luck with your puppers I hope u find some help.
~Friendship is like a bank account. You can't continue to draw on it without making deposits~
~Tegan 9 year old yellow lab~
~Wilbur 9 year old LH cat~
~Mirabelle 18 mos dsh~
~O'Shawnnessey 18 mos dsh~
~Darby 1 year old dsh~
~Mindy 7 yr old shih tzu~
~Dexter 10yr old Salmon (large goldfish)