Is he using private insurance such as traveler's insurance? Or is he enrolled in one of the insurance schemes operated by the Japanese government? If the latter, dental is covered.
If he is not already enrolled in the government plan, he may wish to pop down to his local city office and get signed up. Caveats: This will involve monthly premiums based on his income level and treatment will entail a 30% co-pay from him. Dental treatment under the government plan tends not to be prohibitively expensive. I've always been able to walk out the door without paying more than the equivalent of US$10 - $30.