save our boxer cody

cody came into our lives after 2 months of searching for the right pup kennel after kennel. my husband reminds me all the time of that day. it honestly felt like we had our first newborn baby. it's now been 5 years. we have a 3 yr old son and an 18 mnth baby girl who love cody unconditionally. but cody is sick. he is bleeding from the mouth, his stools are black, he isn't eating but still throwing up. he is so severely sick that our vet does not believe he will survive. but we believe he can survive. cody is so strong so full of energy and love, we can't, theres no way for us to let him go so easily. we are committed to staying by cody's side and fight this sickness all the way through. please just please help us.