I mean this in the best possible way Roughquest - if you read your own question back I think you are answering yourself.
If this is really upsetting you (it will be upsetting your dog too) and there is no obvious cure (your vet would have told you if there was), then there really is only one option.
Your vet would not advise this if he thought there was any other reasonable solution. 14 is a good age for any dog and although he appears healthy in every other aspect his mind seems to be going. If you can constantly smell his own poo on him imagine how he feels? To a dog that has been clean all his life it must be distressing. How are you going to continue bathing him and drying him constantly in winter - when it is freezing outside?
I know it is hard - we have all been there - everyone is different, but I get the feeling from your letter that like I say you already know the answer.
Letting go is always hard, but if you can take a step away and remember the dog he used to be, then look at him again as he is now, is HE really happy? Is there any realistic chance of recovery (or is he just going to get worse)? How do you want to remember him?
You know your lifestyle, feelings etc. and others cannot judge that, everyone has different standards and what is acceptable to one is not acceptable to another. Hard as it is only you can decide.
Sorry, this may not be the reply you wanted to hear, but sometimes honesty is the only way.
Thinking of you.