I don't mean to dampen your expectations or your search, but, be very cautious of blending a private pitching coach with a pitching coach employed by, and used, by your son's college program.
Some, not all, coaches at that level can be very negative with someone bringing their pitchers along - possibly contrary to their instruction and coaching. Other pitchers may pick up on a style and format that serves your son well, then notice that your son is getting coaching elsewhere. The buzz in the clubhouse can go very badly for your boy.
My point is, be mindful of the mindset and personality of your son's pitching coach. He may interpret your son's progress, if any, with some sort of job security issues, a dent in the man's ego and so on.
By the way, many coaches not at the helm, are often hired and brought on board by the head coach, at some point in time. These coaches have to deal with the many immaturity characteristics that are common with college athletes. In that regard, I listened to one of the best introduction speeches given by a college skipper during a meet-n-greet with incoming freshmen and transfers... it went something like this:
"Welcome gentlemen... as you may have noticed, we're meeting at the campus swimming pool, and for good reason. I am standing at this end, and you are standing at that end. When the day comes that you can walk across that body of water, and reach me here... that'll be the day that I'll start listening to you. Until that time, YOU WILL LISTEN TO ME."