I'm a dressage rider, working in first level, which is where extended trot is started.
Before you ask to extend, make a reconizable half halt, and then, push forward, without loosing contact to his mouth. Start slowly, build up the forwardness, get to your highest point and bring him slowly down to a regular trot again. Have a set place where you wanna start and finish, such as a diagonal or long side of a ring.
Hows that sound?