Keep it simple, just the facts. Letter should just say you are resigning, and what your last day will be plus something about holiday entitlement if applicable. Email pretty much as headfirst. You could add that you would be happy to discus it with him if he wants. If most of your contact has been via email then I see no big issue with emailing this.
“I’m not going to call him as he rarely answers his phone.”
I’d just leave him a message then.
no, do you want to stay/ angling for a better offer/ think you might come back one day? If not then be pleasant and factural, email will be fine.
Dear …… please find attached my letter of resignation. thanks for the opportunity of working here blah blah blah…best wishes ……