It is good that you are starting to teach the puppy early not to nip. Here's an article that may help:
http://www.paw-rescue.org/PAW/PETTIP...Inhibition.php
When he crouches, he's inviting you to play, and if he does not accept your "no" then he needs to learn play manners. Walking away and refusing to play is the best approach. Some dogs respond to a gentle tap on the chin (just to get his attention). In this case, though, he might think that means you are willing to play.
Shelties are herding dogs, so it may help if you can find a way to let him practice "herding" safely. Perhaps he has two or three stuffies he can begin learning to gather together in a basket?