hey, im not an expert on halter classes but for getting him more comfortable in horse shows you might want to try and bring him to a show and not bring him in any classes. Simply walk him around, pet him, graze him, and let him chill. This will help him realize that new enviornments don't really have to be scary.
Also, when his attention starts to wander put him to work. I.e. Make him think... make him go in cirlces around you, make him go sideways expecially... this makes him think more of where his feet are going so he doesn't fall over rather than the other thing. Also, bonding with him before the show will also help, taking the 'alpha' position of your mini herd, and he will be more inclined to focus on you.
Best of luck!