It's just a puppy thing. He'll get over it. Our puppy was very mouthy (min poodle) until we got another poodle and now he's mouthy with him.
This too shall pass!
I've found that carrying treats with me when I take the dogs out is a good thing. I've been working on "come" with the treats and that's working. If they lay down and dont want to walk on the leash, I coax them with a treat, and they get up and follow. Move the brain forward.