I agree with blocking the dates, because that way your listing can still be booked ahead for later dates, after it will be available again. I know people planning travel for things such as weddings and graduations often want to book well in advance.
Go to the host Calendar and select each day you want to make unavailable. On the right there are buttons for "Available" and "Blocked". Be sure to hit the SAVE button after you make the change.
If his plans change, you can make the dates available again the same way.
I have had to do the dates one day at a time. Someone may know how to select a number of days to edit availability at the same time.
However, if there is a chance your son will extend his stay, it might be best to snooze your listing after his move-in date. Then you will not have to deal with later having to cancel advance bookings.