Leash-training a young Bulldogge puppy?

I have a 12 week old Olde English Bulldogge puppy. We’re working on leash-training him.

Usually, he is at my heels all day long. Follows me everywhere. He wears a collar fine. He wears the leash without a problem (proudly carrying it around the house). As soon as I pick up the leash, he plops his butt on the ground and refuses to move.

How can I train him to walk on a leash, if he won’t get up and walk?