Your son needs to be “game ready”. How many pitches does it take your son to warm-up? Use that as a guide. You son needs to control his performance and if he's not physically ready to throw a pen, he needs to tell the scout. I would make arrangements for your son's catcher to catch him too (will increase your son's confidence). I would have your son and his catcher at the field at least an hour early. Do whatever it takes to have a good showing. Good nights sleep, breakfast, run, stretch, long toss, etc, etc. Is throwing this pen a big deal?? Not any more so than an opportunity to interview for that once in a lifetime job.
Best of luck!
Fungo