The last challenge is to listen to all of Washington's conversations. I've beaten the game and I believe the few missions after it's over. I've played Bowls with him, but the challenge still says it's incomplete. Have I missed something? Is it possible to miss a conversation with him early in the game and not be able to complete the challenge? Or have I yet to do something I'm supposed to?
Finally got around to trying this.....went back to seq 9 and followed Washington around Valley Forge, had a couple of conversations with him but nothing happened, it still shows in my logbook that the conversations never happened.
"To argue with a person who has renounced the use of reason is like administering medicine to the dead."
Bayou, just to be clear there are two different conversation spots at the start of that Seq 9 chapter. He waffles a bit, walks away, then the prompt shows again. Somebody here said there's also one inside the Whitehouse, I believe he said Seq 10, and he said that scene isn't available in mission replay. I vaguely recall seeing a conversation prompt and hitting it. I don't know if it's missable, just mentioning what he said. You might also check x360a and TrueAchievements if in doubt.
Also you can speak to him playing bowls at the SW of NY after the game is done. Maybe that counts too.