Does he have any other noticeable changes in his behavior, eating or potty habits? Is he on medication or does he have any other health issues?
I suggest you take him to the vet for a complete blood panel work up. There are illnesses that can cause this symptom - hyperthyroidism, low potassium, kidney disorders. The blood tests should certainly guide you and your vet in the right direction. There may be a need for neurolgical testing in addition.
I hope you can get him scheduled at your earliest convenience. I wish the best for both of you!