Besides lethargy which he obviously has, here are other signs of dehydration you can check for:
Symptoms of dehydration . . .
1. The skin loses elasticity as it loses moisture.
This can be somewhat misleading since younger and fatter dogs will have more elasticity than older, thinner dogs. It is important to have an idea of what your dog's skin looks and feels like on a normal basis. Pinch a little skin between your thumb and forefinger on your dog's back. When you release it, it should pop back into place immediately. (You can try this on the back of your own hand as an example) As the tissue under the skin loses moisture, the skin moves back more slowly. In extreme cases, the skin doesn't pop back.
2. The eyes appear sunken and lack moisture.
3. The mouth appears dry. ... gums and nose are dry.
4. Delayed capillary refill time
Pull up your dog's lip and look at his gums. Place your index finger firmly against the gums so that they appear white. Remove your finger and see how quickly the blood returns to the gums (they will become pink in that area again). This is called capillary refill time. If you do this when everything is normal, you will have a basis upon which to compare. The gums of a normal dog refill immediately, the gums of a dehydrated dog could take 3 seconds or so to return to their pink state.
If your dog shows visible symptoms of dehydration, he may need a trip to the vet for immediate replenishment of fluids. In serious cases, your veterinarian may administer fluids under the skin or directly into the blood stream. Dehydration in Dogs