Tips for Showcase Bullpens

Well you could always have him chug a double dose of C4 and chase it with a Redbull before hitting the mound. I wish I was just joking, but that's what the competition is often doing.

To not joke. With experience at the showcases he can learn to pace himself. Its a long day for pitchers. Even longer when you don't know your exact time to pitch during the day. As the day drags on, just stretch, hydrate properly, and try to stay loose. Hard to do sitting and waiting your turn if you are deep on the list, or the weather is August hot, or December 35 degrees. Observe learn and pace yourself to be ready is the only advice to give. Do not warm up 5 times to finally throw the 5th time. If he can throw 84 he can throw 84. or more.

Depends on format of showcase. But in general I'd say because he's throwing maybe 30 pitches? Warmup longer and with more intent. It's not like a game where you may be expected to go 80-100 pitches. No reason to leave anything in the tank. If he can't maintain his velo for a 30 pitch showcase, he shouldn't be there.

Other things from my kids experience;

Find a good player to warmup with, don't team up with a kid that shouldn't be there. 

This goes back to "warmup longer" but make sure he's paying attention to what's going on and has plenty of time to warmup. Somebody is always watching.

As a PO my kid traveled out of state for a showcase tourney and waited all week for his start only to choke.

His turfs were slipping on the mound. His velo sucked. His location sucked.
I don't think he completed 2 innings before he got the hook.
Several coaches - who I believe were there looking at him - shut the door on him that day.
Huge waste of time money and effort.

Life goes on.

He had other, better outings elsewhere. Other schools who were watching him came to see him pitch at his high school games. Some were in communication with his pitching coach.
Some were at the playoff game where his threw a complete game and shined.

Things will work out.
Keep it positive with your son. Try to help him be confident and not overthink.
Tell him you both know he's a stud. He just needs to show them he's a stud.