I agree with DA. The most important thing is to present a relaxed horse doing something he loves. If he's likes the change to English, try it out for a while.
In any case, you want him groomed to the breed standard. Make sure all tack is clean, in good repair, and fits properly. For yourself, dress in whatever attire is appropriate to how you're presenting your horse. Make sure everything fits you properly.
If trail riding is his thing, you could practice some trail class type maneuvers and see if he's comfortable doing that. I know breed demonstrations have representatives of the various things the breed does, so if you're locked into the English thing, I think lessons are a great idea. You might want to do a couple now, practice for a while and do a few more closer to the demo.
If you'd like more specific advice, we need more info. What breed is your horse, and could you post a picture?
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