Dog is dehydrated...any help?

I have a male Lab that is about 9 years old. We always give him plenty of water and spray him down with water on hot days. He is an outdoor dog. Recently he has been staying in his little room that we have outside for him. He usually comes out and greets me at the door but hasnt been doing that. He is also not wanting to walk much and not drinking or eating. So we decided to bring him inside so he can get completely cooled off. We put him in a large cage and he rested. We then was going to let him out to go pee and he decided to stop in my kitchen and pee there!! He urinated a flood! I have never seen him pee so much. Could he be dehydrated or is it something else? Any tips? I have three dogs. And I have already had to take one of my dogs to the vet recently for getting a stick stuck in her mouth, and I took the other dog just the other day because she was limping. I would like to not have to go to the vet again. So any tips on how I can treat him here would be great! Thanks!