Well it sounds like he might get better if he eats. but you said a few times he's old and sort of wish he would just go and be out of his missery. It sucks but sometimes we have to make a choice for our pets sake. For a mammal this is getting a shot from the vet for fish a trip to the vet would only be needed if you want someone else to do it for you. If you really think he needs to be out of his missery here is a site on euthanizing fish.
http://www.wisegeek.com/what-is-the-...ize-a-fish.htm
http://www.fishjunkies.com/euthanization.php