My son lived in the International Hall at college and some of his best friends were from Japan. He visited a number of them one year and really enjoyed himself. I do not recall anything about healthcare insurance at the time of the trip but he was there for only two weeks.
I can appreciate the story of the scaling and cleaning. I once went to the dentist for this work and it took me two months to recover from the pain. I had to change my toothpaste to a sensitive teeth type to help alleviate the problem.
Health insurance is not available for those staying in Japan on short-term, so your son didn't qualify for it. He didn't fall sick then, I suppose? Good, coz without health insurance, visiting the doctor is pretty expensive.
But two months to recover from your pain... wow. It must have felt really terrible!