I'll assume you can not practice in the same arena that caused the issues - so instead you need to address his behavior.
At home experiment and try to find something ELSE that "upsets" him and cause similar behavior - like a flag or a tarp popping in the breeze, ...
Once you have something to initiate the same behavior you can practice (experiment to find) methods to control his behavior and re-direct his attention to YOU where it belongs.
So when horse starts mis-behaving you want to get him close to where the bad behavior begins and start asking him to work on things that get and keep his attention. In dressage that is shoulder fore/ shoulder in, several steps (at walk, trot and/or canter - suggest for control reasons you start with a walk) then once you get a few good steps switch to haunches in/out or leg yield or half pass - you getthe idea.
What you want to do is re-direct his attention away from the bogey man and back to you.
Once you can control him where he INITIALLY misbehaves, SLOWLY move him towards the bogey man.
To give you an idea - one day my mare would NOT walk past a milk carton in the pasture. So I started this technique. Next thing you know we backed up PAST the bad old milk carton.
So instead of her walking past it she backed past it. After repeating it a few more times we could easily walk past the carton.