Sounds like you need to take him to a vet. There can be many reasons why this is happening and it hard to tell you what could be causing this.
Senior Greyhound Has Loss Of Appetite And Losing Weight Fast - What Could This Be From?
Dog could have diabetes, severe gingivitis, a myriad of things.If gums are very pale, get to a vet immediately!
It can be for a myriad of reasons, I e overactive thyroid. But clearly you should be taking him/her to see a vet and get a proper diagnosis and treatment. You would be feiling your dog in his need if you did not do this.