As far as I know there's no real 'guideline' set for letters of recommendation, at least not as far as the ADEA is concerned. I always think of letters of recommendation answering the question "Why should (student) be accepted into dental school?" so as long as he writes something that answers that question it should be fine.
Edit: Unless of course he's asking you for a list of stuff you want in the letter then just make a list of qualities that you think would sound good to the adcoms 😉