It sounds like your guy has never been taught to listen to “whispered” rein commands. The basic idea is that if you normally ride with light but consistent contact, you can teach the horse to listen to your hands so that you need only to increase the amount of tension in your hands & fingers without moving your hands or elbows to instruct the horse to slow down. Conversely, if you lessen the tension in fingers and hands, again without changing the hand and elbow position, that change tells the horse to give you more power from the motor attached to the rear end, which can be expressed as more speed or a longer stride, depending on what other commands you simultaneously give with other parts of your body. I call this “riding the horse in your fingertips” as that is what it feels like. I did a long explanation of this technique in Joe-Joe’s topic “explaining that a show is not a race”, and referred to that post in the “Hot Arabian Mare” topic, where I also listed some additional information.
This is not a quick fix. It takes time to teach the horse to listen to minute changes in your body, hand tension, etc. and it does help if you think of it as teaching the horse to listen to “whispered” commands.