Casting creatures as spells - searching for the rules
We have


  701.13b To play a card means to play that card as a land or to cast that card as a spell, whichever is 
  appropriate.


There is no room in this rule for a way to "play a creature" other than casting it as a spell. Only lands go directly from hand to battlefield. Unless he has a card with an effect along the lines of "put a creature card from your hand onto the battlefield", as those creatures aren't spells.