The horse is too young for ridden work. Please go here http://www.equinestudies.org/knowledge_b... click on "The Ranger Piece" at left. The menu title will change to "Skeletal Maturation" When you click an article in PDF format will open.
The topic I direct your attention to is on page 6 of this document. This article is written by a Smithsonian Class Paleontologist. She is also an accomplished equestrian.
She has forgotten more about bone development in horses than most vets will ever know. Ignore her advice at your horses peril. The fact that you are asking this question instead of going to a colt starting clinic with a well respected trainer tells me you are not ready by half for this task.
Tiny things that are done or not done at this point will make a huge impact on this horses life. I see literally hundreds of horses with problems in their training that could have easily been avoided had a competent colt starter, or a experienced rider under the direct supervision of one, been their initial experience. Beware; there are many stinkers out and about that are not good at their craft.
Final note: There was a time that I would have advised an owner of a young horse to begin the initial training (groundwork) I no longer advise this unless that owner is sufficiently ego free to get their hands on some good audio video material and S T U D Y it, and repeat studying it as they go along.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.