Given your description of how this weanling behaved with the Seller, it sounds to me as if the Seller has never handled the poor thing.
The young lad should've been handled from the moment of birth forward.
You are going to have to have the "Patience of Job" and take his training in tiny baby steps, because he hasn't had any training at all. What the Seller did do doesn't even get on the low end of the scale for training.
Folks are right in suggesting to take your time. Make his lessons short, and fun. Don't expect the same things out of this young one that you expect out of an adult horse.
That saying "learn to crawl before you can walk" applies to horse training -- especially when you are dealing with a baby.
I am not a fan of the NH trainers, but this is an exception. I think you might benefit from getting anyone of their DVD's on colt starting.
Best of luck to you