How do you comfort the broken hearted?

after receiving a call from my dad who has lost his lifetime mate just weeks ago I feel compelled to write a letter of encouragement to him. How do you write a letter to someone who has never known life without my mom? Who comes from a generation that a husband and wife were one and remained together until death? Who all of his adult life has been joined with this woman and known her since her teen years? His only question that he asks all of his children is, "how long will it be before I am with your mother again?" I know writing would be so much better since he won't know I 'm crying. On the phone we both end up crying and then hanging up. How do I comfort him and ease his pain?