dog has poor appetite

Question
My neutered male shepard mix will be 15 in April.  His appetite has decreased significantly.  His sister displayed the same just before she died in Sept.  I realize this is old.  Under vet advice I give him cooked chicken thighs, rice and vegetables. He was animal agressive when he was younger (except to his sister) and I don't think a new dog would be a comfort, even though I know he is lonely. He chases the cats bk to the barn(his job)likes to go for walks, but mostly sleeps. He is free to roam anywhere on the farm. He eats anything from my hand.  To entice his appetite I feed him what I am eating. Am I just prolonging the inevitable?  I tried once to put him to sleep, but could not handle it and cancelled the appt.

Answer
This is a very tough decision.  What I feel is the major factor on when is the dogs quality of life.  Is he still enjoying chasing the cats?  Is he happy just to nap at your feet?  Is he in pain?  As long as he enjoys life, go ahead and prolong it.  if he gets to where he is just existing and is in pain, then it is time.