This is a typical question that has been asked. This can be tricky but I have provided screenshots to where you should be and who you should talk to.
In order for him to establish a cannon in the Courtyard, you must do two of the following things.
If it is nighttime for you, talk to the women in the bar and she will mention something about the cannon.
If it is morning, go to the south west house and talk to the lady near the table.
Now you can go to the courtyard and see the cannon.