Correct me if I’m wrong, but it just sounds like that your horse is hot from not having been worked in quite a long time due to an injury. He needs to get that energy out in a productive way. But that energy is coming from his diet. It appears you should have adjusted and changed his diet accordingly early. If he’s on a sweet feed, (or a feed with high sugar) he would basically be a toddler pumped full of sugar but unable to scream n’ shout n’ let it all out! I don’t think he’s being bad for the sake of being bad, just ancy and bored of being hurt. You could always lunge him at the walk, or use a chain as you see fit. Good luck!