In Person?
I may be wrong but I'd want to see his reaction in person when breaking the news. You can tell a lot from a person's body language. If telling him over the phone, you will be left in limbo and won't have a clue what his next move is.
Of course if this were to be in person, it would have to definitely be in a public area and make sure there's someone you can leave with.
Good luck to you Dear and keep us posted.