If you really want to tell him why you left, you should do it in person, and with only him present. Don't do it over the phone. Elders have been known to pretend they are the only one on the phone, but then have other elders/witnesses listening in on extensions. Put nothing in writing.
They need two witnesses to execute you. If you don't want to be executed, then take my advice. If you don't care, then none of that matters.
Farkel