Our trainers taught us a great little trick for this very problem. As soon as he bites or nips take his face in your hand and with the other hand put your thumb in his mouth (sounds crazy!)and press down LIGHTLY under his tounge. (kind of a light squeeze of the lower jaw) At the same time say "NO BITE!" in a firm (not loud) voice. Then just stop playing with him and walk away. This worked for us better than anything we had tried. If you're unsure about how much pressure to use, do it to yourself. Obviously you don't want or need to hurt him, that's not the intention, just to make him feel a bit uncomfortable. Also make sure to reward good behavior.