Oh dear. It does sound like the beginning of the end. Incontinence does affect their quality of life (not to mention yours). If he starts weeing in his bed for example then I think your only choice would be to have him put down, but if at the moment he can keep himself more or less clean then that's not so bad (if you can cope with it).
Apart from bloods, looking at his teeth and taking a history I don't think the vets will do much more.
19 is certainly a good age. I hope the vets can do something to help him. If not then discuss his quality of life with the vet and of course you will know better than anyone if you think he is no longer happy. And remember the last kind act we can do for them is to give them a good death when the time comes.
Just a thought the last cat that I had with this presentation was actually diabetic. Once on insulin everything improved no longer incontient (well wasn't originally just drinking so much) legs improved he was 18 at diagnosis and live two years with really good quality of life. So bloods could make a huge difference.
The last cat I knew well with diabetes was not only on injections but was terrifically difficult with feeding - a fussy Siamese who had to be fed regularly to keep his sugar levels true. It was extremely difficult for his owners.
Is that not common then? I would love to hear that some (or most) diabetic cats are easily treated.