Hello Mini Luver,
I have been working with driving horses for quite some time now, and I have my fair share of success. If you take the quality time and effort with nearly any horse, they can be a driving horse.
Three years ago, I showed a loud appaloosa mare at the AMHR Nationals in futurity where we won national champion in roadster. When I began working with this mare (only 3 months before), she did not want to be caught, she did not like to be stroked, and she had never been harnessed or driven. I spent the time on her ground work, broke her to lead, got her comfortable with the harness, and drove her to national champion her very first show - safely, I might add. By the time we were finished, the mare was comfortable in the cart and willing to do anything I asked. She didn't have the perfect headset (well, I didn't think so), but she was SAFE. That is most important here I think, she was safe.
I use this anecdote to remind you to never get discouraged. It CAN be done, and never let others tell you that it CANNOT. I encourage you to work with your little guy as often as you can, and slow progression is important.
Also, many horses are sensitive around their tail, my suggestion is to simply rub done their hind end with your hands - often, brush him, comb his tail, etc. Hold the tail bone and move it up and down (sounds silly when I describe it, but it makes sense when you try it). Hands-on desensitizing is highly effective.
Anyhow, you have the right idea, and maybe if you can get some help from Mi-Lo, you will start to see some progression with Wee-Man. I wish you the best of luck, and feel free to PM or e-mail me anytime you have driving questions. If I cannot answer, I will point you in the right direction of someone who can.
Tammy