I also agree Aly's suggestions are very good.

As for the leash in his mouth,
this is what I did for my dogs.

When Rocky would grab the leash,
I would stand still, remove the leash from his
mouth telling him "leave it"
I than would tell him to sit.
I would not put his leash on till he sat nicely and
calmly.
If he put that leash in his mouth, NO walkie.
He caught on after a few times,
and realized he wasn't going anywhere
till he sat and behaved himself.

Welcome to Pet Talk.