I have trained and worked with some really spooky horses in the past. The best way in my opinion is to take him on walks, expose him to as much as you can BUT use another horse that is really confident and not spooky and bring it along. It worked out great for all my horses. Watch your body language while training/working with him, when he spooks and you remain calm and relaxed he will calm down way faster and will become more confident - it might take some time though, so rushing is not the way to go! Oh and I agree, the past of the horse does not really matter that much, just think about the here and now and your horse will do the same!!!