I've just completed the mission to retake the castle for the Minutemen. Preston was my follower before the mission, then left when it started to go to the meeting point.
After completing the mission he returned to Sanctuary and I can no longer speak to him, does anyone know if this is a bug, or there are subsequent quests I must complete to get him to follow me again?
Edit
I've since finished the old guns quest and I spoke to him to collect the reward but couldn't bring up any extra dialog with him.