Read these two free downloads:
http://www.dogstardaily.com/free-downloads
Read the Sticky: The Bite Stops Here in the new owner section.
Your puppy is starting to teethe, and he's gaining some confidence, so he's looking for more interaction and for more independence.
Nipping - Assuming that you've read the Sticky: The Bite Stops Here. Rather than yelping, try saying Ouch! or Oops!. What you need is a word to mark the nipping in addition to ignoring him:
Read this and note the 3 days and the apology....
Some Tweaks to Bite Inhibition (to get him to stop biting when he wants to play):
1. When the pup bites, then yelp. It should sound about like what the pup does when you step on its paw... don't step on his paw for a sample

. When you yelp, the pup should startle briefly and stop nipping. Praise and pet. He'll bite.
2. When he bites the second time, Yelp. When he stops, praise and pet. He'll nip again, although it may be a little gentler. ...
3. When he bites a third time, Yelp (see a pattern?). But this time, turn your back for 15 - 30 secs. If he comes around and play bows or barks, then that is an apology. This is important. Accept it, praise and pet... and cringe in expectation of the next nip...
4. When he bites the 4th time, Yelp, then leave the area, placing him in a 2 min. time-out. It is better if you can leave, rather than moving him. Then, return and interact. (He's still hungry...)
5. When he nips the fifth time, yelp, and leave the area, stopping interaction for now.
Pups need to sleep over night in order to learn their lessons. So, keep doing this for 3 days. By the third day, you should notice signficant Bite Inhibition. He may still nip, but it will be softer and he won't draw blood. Keep up the training and make sure that everyone yelps.... Very powerful method.
If you learn the technique, then you can apply the "yelp" to other circumstances, also. I believe that "yelp" is "Please don't do that, I don't like it." in dog communication. I currently use the yelp when my dog plays tug, then runs with the toy, when he fetches and keeps it out of reach or when he takes a treat too quickly....