This book is excellent for describing the strokes and the biomechanical reasons that these stroke techniques work. Logical and clear. There is also a lot of developmental information for young players.
Technique Development in Tennis Stroke Production
Bruce Elliott, Marchar Reid and Miguel Crespo
I have only found this book offered by the ITF site. Unfortunate, as this book should see more exposure.
I would introduce your son to the simpler concepts that are currently used to generate pace such as the use of the stretch-shortening cycle. Concepts of this type are essential, very simple and can be understood and used to some degree by developing young players. These concepts are well described in the book along with some discussion of developmental age related milestones.
On the less fun side to think about - develop your response beforehand
as to how you will deal with discomfort, pain or injury when it occurs. This is probably the most important issue especially if your son should become serious in tennis. Don't let this issue surprise and pressure you in the last minute before some important team commitment, tennis tournament, etc. Research your Dr before you need a Dr.