We are the proud people of a 15 week old "red heeler" Australian Cattle Dog. He's no "purebred" but his line on both sides is hard-working cattle dogs straight off the farm. His original people separated him from his momma dog at 8 weeks. Mom dog is very sweet and good tempered, so is Dad. Eddie is all heeler. Stubborn and loves to play rough, but very good natured. Not an alpha, but kind of a middle-of-th-pack-let's-play kind of guy. However, not surprisingly, he is a biter. Especially of me, the Momma dog. If we isolate him for biting, he comes back wanting to play (bite) even more. If I tap his nose, say "no", growl, yelp, pinch his cheeks, or do anything else he thinks it is a game and comes back for more. He's only nipping at our family members, or people he knows really well and feels he can play with. He's well behaved around strangers, and even sits on command so they can pet him. He's even good at backing off little kids when I tell him to. I think he sees me as the Momma Dog, so he feels free to gnaw on me at will.
Yes, he gets plenty of exercise, running, hiking, playing with the high-energy dogs in the neighborhood. Boxer, Springer Spaniel, Labradoodles, Australian Shepard and all the other dogs. I'm thinking either a "real" trainer or electronic "helper". I just don't want to see this problem get out of hand or see him become aggressive (although he is too overall sweet-tempered for me to see him as truly aggressive).
I hope this helps, I too have an ACD. She likes to nip at heels. What your puppy is doing is learning through play, how to take of itself. I was in the K-9 Corps in the Army and we wanted dogs to bite, because of the nature of the work. Your situation is just the opposite. Your dog views you as the Alpha pack leader and as such is testing you. Will you react in a manner that is submissive or will you set him/her back on their heels.
Without using physical means such as hitting, you want to re-enforce your position as dominate in the pack. What I suggest is when the pup does the biting, put your hand around the dogs muzzle, and in a stern voice say "...NO...! Bad dog!" I will tell you now that the paw will come up and attempt to push your hand away. Don't let that happen. You are the boss, so let your dog know that. ACD's are head strong. They make a great addition to the family and show their love without conditions.
Try this, if it doesn't work, there are other ways to deal with this, but the are a little more stern, so I would try this first. I would very much like to hear how this works for you. Also you will have to do this more than just a couple of times.