I have never driven a big horse or a mini. Where would I start on my training? I would imagine I can't train him to drive if I am clueless. Also, on regular horses, usually you start riding them at two.
Well, the first thing I suggest is that you take some lessons with a "finished" driving horse. It doesn't even have to be a mini, but if you consider that you probably wouldn't train a horse to ride if you have never ridden, it makes sense as to why you should know at least what a driving horse feels like before you go about training one. I would look for a carriage driving instructor or a breed ring instructor, but not a Draft driver. While some of it is the same, it really isn't what you are going to be doing in the future and you will get confused with the terminology and the methods. We were "led astray" early in our driving "career" by some draft guys that do things just a little differently than light horse drivers.
Then, the next best thing to do is to see if your instructor can 1) either help you through the process of training your horse to drive, or 2) recommend someone who can. There is only so much you can get from forums and books, especially if you want to show your horse someday. If you make mistakes, it will take a professional a lot of time to correct them, which will cost you more in the long run.
In the mean time, work especially on Showmanship type halter work and show him in Halter, even if he isn't a Halter horse. This will prepare him for the stresses of the show ring and teach him ground manners. Horses that have good Showmanship training are MUCH easier to teach to drive. Use the same verbal cues for the Showmanship as you will for driving, such as "Walk, Trot, Whoa, Stand", etc.
You can also work on "desensitization" when they are young, by having them do such things as walk over stuff, have stuff put on them, drag stuff next to them, etc. We have safe "toys" that we "play" with, like old rugs, a piece of at least 1/2" plywood, pool noodles, bubble wrap, playground balls, plastic recycling bags, and PVC pipes. Do a search on this forum for desensitization for ideas on how to go about doing this training. Make sure that you have a very "matter-of-fact" attitude about it though. Don't transfer anxiety to your horse by how you go about it.
Hope this helps.
Myrna