Ask players from the nearest travel organizations and colleges who they work with individually and who they might know about. If you can't connect that route and you have to use a commercial training facility, most have profiles of their instructors on a website. Look at their background as it compares to the skill set you are wanting to work on. From there, you can contact those instructors and ask further questions if needed.
The most variance with types of instruction comes in the way of hitting and pitching mechanics. Since it sounds like he isn't going to focus on pitching, you will want to direct questions to general hitting philosophy. Just ask the instructor what his primary mechanic direction/focus and philosophy is with hitters. See how it matches up with your son's direction.