How about driving by the house you lived in with your first husband? You could just say something simple like "I used to live in that house when I was married to Joe" or to my first husband. . .however it is more comfortable, but where he doesn't mistakenly think you're talking about his dad.

Make the conversation about the house, not the marriage. Make the statement, then move on to "that house had the brightest pink bathroom. I sure hope the people who live there now have changed it." If he's interested, he will pick up either of the statements. If not, the seed is planted and he will come back to it another time.

You're right about not letting it wait until suddenly he is 16 and still doesn't know. Lynn is so right about not making it a somber moment. . .just use a teachable moment, or make a comment in passing. Kids hear everything. & they remember everything. He may not even show any interest right now, but if you mention it when its handy. . .not dwell on it, but move the conversation on, that will give him some room in choosing when he wants more info.