and I do think he led with his head.. It could have been called.

However, you CAN just continue a shoulder block on a player in the manner he did (even away from the play). What rules prevents such an effort to block an opponent like that? It doesn't matter that he's on the ground. Once you pancake a guy you can continue to block him. You don't see it often because the smarter thing would work your way downfield to pick up a block on a potential cut back or something.