So i have a german shepherd. when i try to walk him he jumps and bites me. not playful little nips either, like jaw clamping down on my arms hard. he hasn't ever broken skin but i'm covered in bruises. it's hard to get him to stop since he's jumping while doing it. i'll step on the leash to prevent him from jumping, but then he turns to biting my legs. what should i do to prevent him from biting and jumping in the first place?