My mare is very un focused and stressed so I usually use an oral calming paste such as SmartShip and Show paste. It’s legal and the horses love the taste. It provides digestive health, electrolytes and antioxidants.
Hi, my mare usually acted like that. What I found the best is to ride and lunge as much as you can, so to do this I recommend you to arrive early in the morning before the first class of the day starts and so show her around an the environment. Also if you can take him to schooling shows and little by little take him to bigger and bigger shows, with more people and horses. I hope this helps. Stephanie
Maybe try taking her to some shows where you aren't competing. Walk around and reassure them that's it's nothing to be nervous about. Then once they learn to settle ride them in the busy warm up like you would at home. Start with small local shows and then once you both can handle those, grow to bigger shows.
You can get oral calmers that are legal but you may want to check with your show rules and their prohibited substances.
Other than that I'd recommend picking a quiet area to warm up in away from everyone, it might help if you stick to a warm up routine that he's used to that will calm him down.
After that it's up to you to remain calm and relaxed! Hope this helps!