I think it is time to get help - someone with the right qualifications and experience to work with you on this problem. Practically all pups play bite, and the odd nip that breaks the skin is to be expected, but regular, frequent, forceful, deliberate biting is not. It sounds like a game that has got out of hand, and the longer it goes on the more difficult it is going to be to sort it. I'd find a trainer now to work with you one-to-one - someone who will visit you at home and see how you and Leo interact in different situations - think of it as an investment in a happy relationship for all the years ahead.
To sit with a dog on a hillside on a glorious afternoon is to be back in Eden,
where doing nothing was not boring- it was peace.
~ Milan Kundera