Something you might try to cut down on noise from the keybed going downstairs is isolate the keyboard stand from the floor. Rather than the actual sound of the keys, what he’s likely hearing downstairs is the vibration of you hitting the keys traveling down through the stand into the floor. Some heavy carpet under your keyboard can do wonders to quieten things up downstairs. Plus maybe some foam between the keyboard and the stand itself. Push comes to shove, try putting a towel under the keyboard to absorb the shock.

I honestly doubt he could hear the noise of the keys themselves, but it’s pretty easy for vibrations to travel through metal stands into a hard floor and through to a ceiling below…