Be truthful. It may hurt him but he will get better over time. Try to stay his friend if you can. Me and my x that i dated for about 2 years are still good friends. Yes it hurts when you break up but time heals. I'm getting married now in a few months and my x is happy for me. I might even make him my kids (when i have kids) god father.
number 1 rule be honest with him at all times. number 2 offer to be there any time he needs you to talk to and be understanding if it takes him a while to get over you. It only hurts when the person instantly changes on you and then moves out of your life