One of the very best things you could do with a young horse to get him used to a lot of things is to use him as a dude string rental guide.
When we moved to the mountains I brought my very green 5 year old horse up after he had been boarded at a relative's house for a year with no riding and only about 10 rides on him before he was boarded.
I got a summer job at a rental stable and took my gelding to use as a guide horse. He was pretty excited at first, but the dude string horses plodded along in a single file line and he got used to either riding at the rear of the line if we had a big group, or riding along side the string of horses if we had such a large group we needed 3 guides or leading the group. One day I was leading and a little girl on our one and only pony was behind me. The little girl started screaming each time we went down a little hill because the pony would speed up. So I calmed her down and would go down the hill very slowly, then stop and let the pony ram into my horse to slow the pony down, then take another few steps and the pony would bump up against us and couldn't gather speed. Now, you have to know your horse before you do that, but I could tell my horse could handle it and by the time the summer was over I had a really smart trail horse that could handle just about anything.
