A good way to stop the behaviour and not spoil playtime is to encourage him to carry a toy during play. If he does mouth your hand shout, "Ow! Ow!" and pull your hand away. Turn your back on him for fifteen seconds and then resume play and again encourage him to carry a toy. Do this EVERY time he mouths you and get everyone else who plays with him to do it too. He is only playing and needs to understand that you don't like it.