Fatty sounds like he’s a big part of the family, much the same as my dogs are…… and I know all you want to do is help the little chap out but like others have said if he’s just slowing down but is still happy then stick with him and be the family he knows and loves till he really is ready to go.
Maybe now is the time to repay a little of the joy and happiness he has brought you over the years by helping him out abit and putting up with some of his old age cranky-ness!!
Do what, in your heart, you feel is best for the little fella and for the family as a whole……
All the best