I hope the tumor has been checked by a vet. Duke is 12 and he has one. They are quite common in older dogs. Duke's is harmless. Also for the arthritis, there are plenty of options. You need to talk to both your parents and your vet. Unless the dog is in pain, he shouldn't be put to sleep just because he's old. Plenty of dogs can function well without seeing. They just need a little extra loving and attention is all.