The horse you describe appears to be an excellent prospect for upper-level dressage and, ultimately, piaffe. It is critical to work with a healthy, cooperative animal. It will be much simpler if he is attentive to the leg and a forward thinker. Older horses may not be as spooky or nervous about new things, but that doesn't mean they aren't intelligent or that they can't learn. When you are teaching an older horse dressage, you will need to start by building his confidence. This can be done by using quiet, respectful pressure on his mouth and legs until he feels comfortable with you. Avoid shying away from him or kicking/punishing him when he resists you. Only kick or punish him if it is truly necessary; don't do so just because you can. Youngsters often think they can outsmart the teacher by being difficult, but this only makes their training harder for them later. If he tries something aggressive, back off instead of reacting defensively. This will help him understand that there are times when you must withdraw respect from him in order to get what we want from him. As he gets used to you, you can begin to show him something beautiful. Start with small movements and simple exercises until he is relaxed around you. Only then should you try jumping or pirouetting. These activities require more strength and balance than most older horses possess so be careful not to push them too hard at first!