My dog is suddenly losing a lot of weight. On top of it he is itching non stop,:cool he also drinks a lot of water... what is going on with him ? :)

With the thirst, my first thought is diabetes. What I think is much less important than what your vet will think when he examines him. Even if somebody here could identify the problem without examining the dog, it likely would be nothing you could treat yourself.

I agree with labman. You need to get your dog into the vet's office for a complete health check. Weight loss and excessive thirst are something that requires medical attention.