Hi Alexa
sorry its taken me so long to reply!!
He is a big powerful horse and this is the one really negative point. I was hoping to perhaps have found a possible solution.
Well clicker training is probably just what you need! Two good books i recommend you get are clicker training for horses by Alexandra Kurland and Don't shoot the dog by Karen Pryor. Like you said your horse has bad memories from somewhere before you owned him as long as you have taken the steps to check there is nothing phycically wrong with him clicker training is what most defiantly will turn his life around.
To begin with you need to condition the noise as a noise your horse will like hearing, usually we use a target that the horse touches to make us click with gets him his treat. I'd work away form the things your horse doesn't like first of all then slowly introduce it to make the things you've decribed more enjoyable. You will be clicking and treating a lot at first but because this way is so positive your horse will become more positive too.
I've been clicker training quite a while now and it never ceases to amaze me what it can do.
Look forward to hearing how you get on!
Becky