He does wonderfully for me with everything except reining (which is really what I'm asking for help on here.) He only doesn't stop for other people, so I guess my answer to that problem is to stop letting inexperienced riders on him while he's still a baby/learning.
Exactly. A young horse needs super consistent riding until they have all of the basics down. Even then, I would only let people ride him who know how to use your cues. It can take a few years for a horse to get really "solid" in their training.
I'm dealing with this myself with my 5yr old Haflinger gelding. My daughter has been riding him and one of my students has also been working with him, and he's getting a bit frustrated by it all. I am now working with him more and more actively helping my daughter and he's starting to get a lot better.