Years ago I had a horse who stayed in a riding school for a couple of weeks while I organised his move to his new home and they used a loose ring bit that pinched and split the corners of his mouth.  I used a hackamore while it healed and it was brilliant.  He was quite strong and in my mind it scared me silly out on Salisbury plain on a horse with no bit in his mouth, however I needn't have worried as he was very easy to control.  One word of warning though -take advise when you first fit and use one an I did it up too tight and as soon as I picked up the reins he threw his head up. Get someone who knows what they are doing to fit it and then ride quietly in a school or field until you and the horse are happy.  

you could try it bearing in mind english hackamores are around £40.00. I would take him of the curb chain and kimblewick, and perhaps put him in a cheltenham or dutch/continental gag. I would not suggest a german hackamore for they are quite harsh and have been know to easily break horses jaws with enough leaverage. I personally dont see how people come to apply so much pressure on the horses mouth for if your horse puts his/her head on the ground to run off you wont be able to stop him putting up a fight or not, horses are far stronger and people who ride heavy handed like so should not ride at all. I suggest trying different bits and if he runs off/or is too strong circle him until he gradually slows down/listens. Talk to your horse or sing, he'll appreciat it:)