At certain points in the year, there will be issues on the forefront of your student's mind. August is all about transition. Whether your student is a freshman or senior, each year is a new one, and everyone gets the chance to start over with a new academic year. During November, your student may be getting burned out. Talk about things in their classes that are exciting them and ask about how they are preparing for final exams. February and March revolve around finding a place to live for the upcoming year. Discuss whether or not your son wants to live on campus or off, alone or with roommates, etc.
Regardless of the conversations, always ask open-ended questions. Questions that require a yes or no answer are going to get a yes or no answer. For better conversations, ask about things that are going on in their lives and on campus to give you more common ground to talk.