*tooth decay (or something wrong with his teeth)

Unlikely to cause weight loss and anorexia in a dog only 2 yrs of age.

*something stuck in his mouth

Open the dog's mouth and look for possible tumors- check under the tongue.

*something bit him in his mouth (Cocoa is known to eat any kind of insect or bug) This could cause swelling of his throat which could make him unable to swallow well which could cause drooling and him being unable to eat. He might be drinking a lot of water because the cool water feels good on his throat.

Are you saying the throat is swollen? If so then check the dog for other swellings, as in possible lymph nodes. Again a bee sting would not cause prolonged loss of weight and eating problems.

The chances are the dog has a serious problem either metabolic or possible cancer. If all of the local Vets are uncooperative and friends of the family can not help, then contact the ASPCA or a local animal rescue group. Explain to them the financial situation, and that the dog is suffering- ask for their help in at least getting a diagnosis and some minimal treatment. Some rescue groups will take a dog with a serious medical problem, treat it, and then put it out for adoption.