I had the same problem you are having when looking for my first show mini. I went to 2 different farms in one day. I was looking at a potential driving horse at one and the one that was broke to drive with a show record. While at the first farm I fell in love with a foal who was reserve national champion down at nationals, and was bred to be a future driver. I REALLY liked him. We then went to look at this other horse. If you went on looks and even personality the foal had me won hands down. In the end I bought the one already trained. No regrets!!! It was the best decision. I think you will know when you see them, trust your gut feeling. He taught me so much and I didn't have to wait 3 years either.
I am from MN, so can't help you on finding a trainer. Sorry. Hopefully you can find a trainer that is willing to work with your son. I don't know if this helps, but even if you can find a large horse trainer that drives in the area, they could help get you started? Hopefully though you can find some mini people close by. Maybe you could check with AMHR/AMHA? Or post an ad here?
Good Luck.