From the rulebook:
2-27-3.b
"A holder is a player who controls the ball on the ground or on a kicking
tee. During a scrimmage-kick play, he remains the holder <span style="font-style: italic; font-weight: bold;">until no player</span> <br style="font-style: italic; font-weight: bold;"> <span style="font-style: italic; font-weight: bold;">is in position to make the kick</span> or, if the ball is kicked, until the kicker
has had a reasonable time to regain his balance."
(emphasis added)
While no rule in the book allows a holder to be down, it is implied that he is. Therefore, a Charlie Brown type play is not allowed as the holder will no longer be the holder and has a knee down. So as long as the kicker remains behind him, he can have his knee down.