I'm very sorry to hear about Dobe! However there is ALWAYS a good chance he'll just make it through operation without developing any new ones later and will be with you for many many years. Especially since he's so young.
Person at my old barn had a gelding (8 yo) developed melanoma on head. BAD one, I mean absolutely horrible looking. All vets and people said it's waste of time, but she was cleaning it every day, and made him through several operations to remove it (in fact he wasn't even her horse, eventually the owners just gave him to her forever, because they had no interest in very sick horse). Guess what - the last time I asked about him I was told it all cleared completely and showed no sign of cancer (to the BIG surprise for the vet), and he's healing well. So NEVER loose the hope!!!