Whats wrong with my dog?

my dog has this huge tumer like bump on his throat and he is rib thin and when i pet him i feel his spine. he wasn't like this about 4 months ago. the thing on his throat is soft so i'm not sure if it is cancer. he is always gagging and couching and can't eat his hard food anymore. i am bringing him to the vet but i wanna know whats going on befre its too late please help me! his name is otis and hes about 12 years old.