older Shitz Tzu licking behavior question

My 17 year old Shitz Tzu has been compulsively licking for more than a year now. he is also sleeping with his tounge out. Could these be habbits be due to tooth decay? He is really too old to be put under anithesia for a teeth cleaning anymore. I don’t want his teeth to be extracted when he is not showing signs of mouth pain that I can tell? He is loosing his hearing and sight but otherwise is spry and lively.
I provide water for him in bowls upsatirs and down all day and night and I have asked his vet who does not no the cause of this behavior. Can you help please with any ideas of what I can do, with great appreciation!