My 18 week old Doberman Pincher bites at mine and my BF's hands, feet, legs, clothes...whatever he can sink his teeth into. When I try to stop him he becomes more aggressive and bites harder and even makes me bleed. I try to get him to submit by holding him down to the ground OR by grabbing his muzzle and staring him in the eyes but he's not having it. He also growls to the point where his teeth are showing. Help!'