Number one, learning how to increase the pace at which the horse was moving. Yes, he had caused the horse to move, but it was moving at a slow pace...and a lot more.
..As said, he wouldn't learn everything in one day.
Aiden was on top of the horse, and it was still moving. He had fully balanced on it, he wasn't shaking at all. One thing had been learned, how to balance on a horse when it's in motion.
And again, he was the one that put the horse in motion, so we can say he's learned two things. Ethan was still glancing at him, and after the passing of some seconds, his voice resounded,
"Now, you've learned how to balance on a horse while it's in motion, hope you'd be able to do the same when the horse is moving at a fast pace. But it's a gradual process though,
Now, why don't you try increasing the horse's pace? It's moving at a slow pace now, try increasing its pace. Remember what I told you while relaying information to you, try to implement it".