The hire my daughter is leasing has that problem too. After cantering all he can think is go go go! And jumping is more like trying to make him stop because he won't listen.
I would google jumping exercises (then click in images) and you will get a lot of twisty paced out courses. These are awesome in a quick horse because they have to slow down in a grid. The ones where you go almost to the jump but then around are also really good because the horse will realize he doesn't have to go fast because he won't go over the jump. Once he calms down, then actually jump it.