I see this is kind of an old question, but in case it helps anyone else: absolutely the best thing I did for myself as a first-year elementary music teacher was to go watch master teachers in action. I learned so much about style, pacing and management that could never be conveyed in any book from watching teachers who were the best in our field. He should try to watch a teacher whose classes are similar in size and demographics to the ones he teaches.
No one writes about managing a music classroom anyway; it's completely different from any other kind of classroom. The advice I would give to your son is that a well-designed lesson prevents most problems. You need to plan to change activities, or at least your approach, every 5-10 minutes depending on the grade level. Fourth and fifth graders can hold it together for longer if the activity is high-interest. Plan to break every task down into manageable steps, and to include a singing, instrumental, listening and movement component in every class. Be very explicit with expectations regarding instrument use, and teach and re-teach routines in every class, particularly with the younger grades.
Your son should expect to work HARD. Fair or not, students come to specials expecting to have a break from the regular classroom and be entertained. I am a non-stop singing, dancing, drumming, xylophone-playing machine from 8:45 when my first class walks in until 2:50 when my last one leaves, with one quick breather for lunch. Elementary music by its very nature is mostly comprised of teacher-led, large-group activities. Rarely do you get to give a short lesson, have the students break into small groups and do their work (sometimes in the upper grades, but in the lower ones, forget it). You have to be ON all the time. Your son will be exhausted, but he will get used to it and come to enjoy it.
Teaching music to elementary kids is a wonderful, rewarding profession and I wouldn't do anything else. I hope he enjoys it as much as I do.