Poles. Toss some poles in the way and change up the pattern you trot over them. After a few minutes of this it usually settles them and focused them on something else. Once you can feel the settle praise praise praise and go town to a walk and let him relax. When he puts two and two together for walk trot transitions toss in a canter transition slowly. For the poles I usually set up a whole course with 2-3 poles at each standard and sometimes one in between those poles on a line.