Well, if you're getting positive results from this trainer, I would work with the trainer more once you bought him. Take some lessons from him and ask him how he was able to build his confidence.
I think that building confidence in any horse just takes time and patience. Don't make a big deal out of things, and eventually they realize that there is not much to be afraid of. Don't push them too far, too fast. Reward them for good behavior rather than punish them for spooking or other undesirable behavior. You have to be a confident rider, handler, and person to handle an unconfident horse. If you don't that you fit that description, I wouldn't buy the horse. You'll just be frustrated with him and fear him in the end.