The JV level should be the forerunner of things to come with the varsity experience. Coaches should be in close proximity to one another - up and down the competitive ladder, communications wise, so to transfer real talent to the varisity system. I use the word system, because that’s exactly what a JV club is for.
Hence, your son’s prelude to arriving at the park, never mind the pen, should include an itinerary of sleep, nutrition, check-in procedures - whether he’s on the school bus or some other method of arrival, then a physical “prep” and clearance by the pitching coach, or some other coach, that your son is physically ok, then an assigned catcher who is schooled in some sort of pen duty “feedback system”, that advises all the coaches just how well - or not, your son is doing for game time. All in all, what I just described should be a reflection of the varsity level experience - so, your son has been introduced to things that won’t come as a surpirse when he moves up.
By the way, this is not magic stuff requiring a Phd., no sir, it’s game simple game savvy that accompanies a coach from the interview process to the field.
Bullpen routines are not stand alone. There part of a systematic check list, occupying a line item with sub routines of “things to do”. And the most important part of that routine, is the feedback to the coaching staff of what your boy’s doing. If your son is not having a decent stint in the pen, then " throwing him to the wolves" is very non productive as well as unhealthy. That’s why rotation schedules are established for sloting in another pitcher if need be.
I understand that coaching is not very specific at some JV levels, and with resources being in a feast or famine cycle, all concerned try to do their best to provide a safe and enjoyable environment. I hope your boy has an enjoyable baseball experience.