I'm no expert but -
No harm in googling up a bit before doing the lesson. Read a bit about the basics - pitch, roll, yaw, and the main instruments (Artificial horizon, altitude, orientation, speed, climb rate, yaw indicator.).
It just means he can concentrate on enjoying the first flight and getting used to the feel of the controls without having to worry about the theory and instruments at the same time.
If he is interested in doing more flights after the first one, then there are standard text books available usually wherever the lessons are.
If he's really into it then could even get MS Flight Simulator, which is also very good for learning the theory and instrumentation. This can save you learning time (i.e.: money) if you want to go on to get a licence.