We just finished our last showcase last October, and many were PBR. We started the summer, before he was an in coming freshman. He's now a 2019 HS grad and signed his NLI with a Big West Conference University last November. You're heading into a fun time in yours and your sons life. As you already read, it's exciting, scary, nerve racking, etc...
And there's great advice already posted above. My advice is for the parents:
1) When your son's out there, make sure he's your focus. Put the cell phone away, unless, you're texting your spouse with updates. Nothing is more important then your son at that point in time. Your son doesn't want to finish taking a couple balls yard, and look in the stands and see you chatting on the phone and not watching him.
2) Setup your equipment early. I had my camera setup and left it running. Then, I'd edit the video later. Nothing wrong with cell phone cameras, but, I've missed too many plays because I wasn't ready fast enough with a cell phone camera.
3) A couple of times, I used Facebook live and streamed the event when my son was hitting, fielding or running the 60, since my wife, his Grandparents, etc. couldn't attend all the events.
4) Make sure you have plenty of wood bats, if it's a wood bat event. My son would bring 2 in his bag and 4 backups we'd leave in the car. He learned that if you break a bat, and don't have a backup, you may not get someone to loan one to you. Now, you're kid is SOL on hitting.
5) Before and after the event, don't be asking him a bunch of questions or being "dad coach". You know, drilling every hitting tip, fielding tip, running tip into him before the event. He's got enough pressure and nerves, he doesn't need you reading baseball digest to him. Just let him know he'll do great out there, and leave it all out on the field. At the end, all encouragement, offer to get something to eat or drink. He needs to decompress from the whole thing. Don't lay into him with a bunch of "you should of....".
6) Each showcase will get easier, mentally. Once the fear of the unknown is behind him, it gets easier. That's a huge reason to get some showcases under the belt early.
Lastly, keep records of how each of his tools are developing(velo's speed, etc). This gives you both a benchmark to see where he sits, and what he needs to improve. For example, if he's an outfielder, and his first showcase 60 yard is 8.0, he'll need to bring that down. Down to what... depends on what level, school, etc. But, it gives him goals to strive for.