Hello,
It sounds like infection, which can cause lack of appetite and stomach upset such as you are hearing. Of course it could also be a underlying condition such as diabetes. Either way I would have a urine test run and a complete blood panel to rule out any other problems. At his age it is always good to have a blood test every year to catch any problems before they become dangerous.
Walter
Its wonderful that blood work was done in September, sounds like your vet is on the ball and prepared to watch out for conditions that like to suddenly appear at this age. Though we also know that while his test may have been great, sometimes these conditions can come on suddenly. I do suspect it is a simple UTI (Urinary Track Infection) so what I would suggest since he just had blood work done is to do the Urine test first, and if there is no infection present then move on to the blood test. I suspect that no blood test will be needed as this is likely the UTI.
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Breeder
Breeder and pet health consultant. Have spent a number of years with dog rescue, training and health