We hope to see you online soon!
He's already used other controls I believe, so It isn't certain buttons on his keyboard.Superheavy I would say don't worry about it too much as it is very buggy and slow. I wish I could disable the prone but I can't. However if you wanna fix yours it might be your keyboard itself. You can also try joining another server that has prone like a Military RP of some sort as they usually have it enabled and see if it's still broken.
We hope to see you online soon!