He's too old for puppy biting IMO, it's become an undesirable behavior now. Has he been through any puppy or obedience classes? If not I would highly recommend it. If he bites you when you sit on the floor I would immediately give a firm NO and get up and walk away, and totally ignore him for a few minutes. Definitely do not sit there and allow him to nip at you. If he persists and follows you around trying to nip you, then I would put him on a 10 minute time out. As far as the leash, I guess I would try coaxing him with treats. Start with just giving him a treat for looking at the leash (you could do this with a clicker too), and slowly work your way up to hooking it to his collar, dispensing treats as you go. You may need to be very patient.