Haha, just get used to the butt cramps, they're a sign that you're sitting in the right position. I almost always get them when I ride.
Your horse looks super cute in western attire. I don't think you'll have too much of an issue getting him to adjust. To me it's almost harder for the rider to adjust than the horse. Just make sure to read the rules on any classes you enter. Usually only "green" horses (i.e. Those under 4 or 5) are allowed to show in snaffles.
Good luck! You two look like you make a good team :)