I agree with what she said.  Practice with him on a lead, moving away from you.  Move at him with 'energy' (dominant body position), maybe cluck at him so it's clear you'd like him to move, and if he doesn't yield, WHACK.  Make him HUSTLE when you move him away too.  No feet dragging, nothing sluggish.  Get him to where when you ask him to move in hand, he MOVES, no questions asked.  Forequarters, hindquarters, both, alternating, flex his head around, ask him to drop his head.  Make him yield every part of his body immediately when asked.

That may or may not translate to the round pen, but you have to learn to read when he's starting to act stupid.  I wouldn't hesitate to lay a hard one across his ass a couple times so he gets the picture.  With stuff like this timing is essential, as is not getting flustered or angry at him.  Be a clear, fair leader, but don't give him any slack. 

You can do it!  Just keep at it, and he'll come around.