Also, (and I do this all the time) instead of trying to make him listen while you're going lap around lap in the arena, go work in the pasture, so you won't have a definite area to work in. Or, do serpentines, triangles, figure eights, etc. make your ride one huge dressage test so he'll have to listen to know where you're going. I've also found it helpful for some horses to talk to them. It makes them listen to you and that way they'll concentrate on you too.