This is probably not the response you are looking for. Your dog, at 15 yo, has lived a long and happy life. Once a cancer goes systemic the prognosis is not very good. Yes, you might buy a little bit of time but the result, at the end, will be the same. We all like to do everything we can for our pets but sometimes it is not in the pet's best interest and can be financially ruinous for the owners. My heartfelt response is to enjoy your pup for the rest of the time he has left and when he tells you he has had enough, the best, kindest, most humane thing you can do is ease his burden and let him cross the Rainbow Bridge. It will be one of the hardest decisions you will have but the right one for him. He will not be alone as 4 of mine would be there waiting for him.