I meant just feed a scoop of oat based feed once a day instead of all the other stuff? And enough haylage to keep him entertained, and keeping him in and riding him every day...
keep him in all the time and don't overdo it on the hay, just give him enough to keep him entertained
just feed oats
ride every day and get him as fit as you can, if you're giving him a few days off keep him in and get him brewing
Having a long line of canter poles (or a canter pole 2/3 strides away) in front of the jump helps with regulating the canter
Also do lots of flatwork and transitions from trot to halt and canter to halt then back from halt to trot then halt to canter
Have you tried draw reins? They could also...