Thanks for the question and welcome to the community.
What you need to do with your little guy is to shape the absence of the innapropriate behavior. If you are having biting issues, my Puppy Biting video can be very helpful because it will show you how to teach him to have a soft mouth, how to drop things (important for your situation) and also the rules of tug.
Try this:
- Hold the leash two feet away from him
- Say, "Yes" and treat or 'click' and treat if you are using a clicker
- Move the leash a bit closer and while you are moving it say, "Good boy" until you are a bit closer and then repeat either yes or click and treat
- Continue moving forward and if he tries to bite it IMMEDIATELY pull it away and say, "Eh! Eh!". This should startle him a bit and stop his movement towards the leash
- Then say, "yes" and treat
- Continue working until he sees the leash come towards him and he looks up at you for a treat
- When this happens, you have changed the behavior pattern to a desired response
- You can also time him out in the crate if he continues to bite for 20 seconds
- When he comes out of the crate continue marking the correct behaviors with a "yes" and reward
Let me know if you have questions, and good luck.
Jeff