Does anyone know the solution to this:

I’m using my s90es as the master controller in master mode to layer a sound from an external keyboard. I am using an expression pedal as a volume control for this sound. The issue I am having is when I select this program in master mode, the s90es brings both sounds up as it should but the volume for the layered sound is at full volume. I want the layered external sound to be zero not full up. How do I send a volume 0 control message to the external keyboard from master mode? I’ve already tried to set the preset volume to 0 but it has no effect. Any help would be appreciated. Thank you.