Is he losing weight as well? Also, has he suffered any bloating round his abdomen? Check his breath too to see if it smells unusually nasty...like urine.

If so, kidney failure caused by old age might be the reason. Not what you wanted to hear really but excess thirst and urination is a symptom of that. Our Jess's sire died of Kidney failure...I won't forget the symptoms of that.

There are other possible causes mind. I'm sure you'll find out soon enough after your visit to the vets.

Sorry to hear he isn't well. Once they reach a certain age, you know something dreadful could happen at anytime and it isn't nice. Whatever happens, I hope he either gets better or goes without too much suffering. Good luck.