Try just taking him to a couple different barns with arenas set up outside. Most barns either charge nothing or a low ring fee. It's worth it to practice.
There isn't any guarantee that the horse will react calmly to the old stable. You could even take him there, though.
What we used to do with a horse that was a little 'iffy' about going to new places, is take him to the show grounds (or the stable where the show will be) and board him there a couple days before the show.
Bu we also had a routine for places they'd never seen before. Haul in as soon as show mgt allows, and take the horses out and start hand grazing them all over the show area. Lots and lots and lots of EATING. Eat, eat, eat. Graze around all the rings, graze around where the classes will be, graze around the warmup areas.
Then ride in all the different rings and warmups. A little time riding in each. Ride all over the show grounds. THEN MORE EATING...LOL. AMAZING how that works, LOL!!!
An especially good place to have them eat is all around the judge's box, so they see it from all angles.
When you go into the ring to ride your test, trot around the outside of the ring for as long as you can, do a few little tiny steps of leg yield to one side or the other, if the horse needs to keep his mind occupied, move along at a determined speed and don't try to hold the horse back. Stop at the judge's box and WALK past, say 'hi' to the judge - that always makes a person relax. And immediately pick up the trot again.
Why walk past the judge's box? Well, for one, you throw less mud on the judge. For two, as said it relaxes the rider. For 3 it relaxes the horse. But instead of doing a lengthy walk, you immediately pick up the trot again so the horse's attention stays on you. Then when you go down the centerline, think of that halt as just the same thing, going to just pause a moment and then go on.