My horse speeds up before jumps?

I have a 15 year old off the track thoroughbred gelding and when we trot to a jump, he always speeds up and ends up cantering a stride before, even if the jump is 12in. causing him to over jump it like it's 5 feet , witch causes me to land on his neck. Even if I give him half haults before the jump he will still go into the canter, or refuse. Any suggestions? Thanks (: