My guest booked for 19 days and was apparently charged half the booking rate. He wanted to know when the second half would be due. Any help with this answer would be appreciated
This information is communicated with the guest at time of booking.
Looking at your listing the booking form shows a second payment is charged about 14 days before the check-in date. The guest will receive a reminder three days before charging (which information i took from the Help screen).
If the guest couldn't be bothered to notice this information on his booking, which would have been provided by Airbnb, it's highly likely that the guest didn't read through all your listing info, either. It might be a good idea to confirm with the guest that they did thoroughly read the description, house rules, etc.