Oh dear. At that advanced age, it sounds like kidney failure to me. You'll need to take your JRT to your vet along with a fresh urine sample. Your vet will likely want to do some blood tests as well to determine how far along the damage is if he suspects kidney failure.
Please don't wait on this. If he's in the early stages, you may be able to save what function remains and slow the process. For now, start giving him a plain Tums tablet either by pilling him or breaking it up and mixing it with his food. You want plain Tums that are only calcium - nothing extra. Calcium helps the body to get rid of phosphorus which is deadly to animals and people in kidney failure. It won't harm him to go ahead and start the calcium before you get a diagnosis, just make sure your vet knows so he can take that into consideration with any test results.
Here is a link to my kidney failure "bible" : http://www.dogaware.com/kidney.html
It amounts to nearly 50 pages printed, and IF this turns out to be the case with your dog, the information is invaluable. It really helped me help my dog who died a year ago from kidney failure.
I hope this turns out to be a simple urinary tract infection, but given his age kidney failure is high on the list of what may be happening. Please do post back and let us know what your vet thinks.
The two things that pop into my head when an older animal starts to drink excessively and urinate in the house are kidney failure and diabetes, although there can be other reasons. A trip to the vet is definitely in order ASAP. If you catch it early, kidney failure is something that an animal can live comfortable with for quite a few years, so don't panic. It can be controlled nicely with diet and the administration of fluids, but first you've got to see exactly what it is you're dealing with, so make an appointment as soon as you can to have bloodwork done to see what his kidney values are, and then the vet can take it from there.
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