Your friends house doesn’t have a PA system, at least 50 random people, some with yappy little dogs or strollers, and he’s not tied to the trailer for hours or in a warm up ring with all sorts of horses doing all sorts of things or the show ring with 15 other horses doing the same thing trying to go slow.
We aren’t trying to argue with you, just trying to tell you we have worked with many horses in the exact same situation and the only way you get them to relax at a show is to go to lots of shows. You don’t have to enter anything, just hang out, ride around the schooling ring and stand around the gate (without being in the way, if course) walk around.
Make it boring. For both of you. It has to be frustrating you, unless you are a robot, when he heats up, that just feeds the anxiety back to him and turns up the heat.
Its a real common problem. But fixable, just not with a bit. BTW Kimberwicks can come with different mouthpieces and degrees of curb action depending on the length of the straight part of the sidepiece above the mouthpiece compared to below it. Called a curb ratio. 4:1 wound be stout, 2:1 pretty mild. If the reins are in a slot, its stronger then if they are sliding on just the curved part of the dee. They aren’t all the same even if they look similar. Bits are an ignored art these days…