Hi there! I wouldn't worry about what the judges are looking for. This should be a schooling show for Oliver, so make it a good learning experience for him. Think about what does Oliver need to come away from this with a positive memory, ulitimately.? You say he needs to build confidence. So, make it that you are very careful in what you ask of him, so that he is able to do it with success. This is really vague, but you , as the rider, would know those places where you can aske a bit more, or you can accept a bit less, for the time being, and not add more stress to the horse's mind.
One thing they look for in training level is that the horse not be behind the vertical. And that the geometry of the circles be good. And like you said, the rythm be steady.