You should start training to the leash with training to the collar.
Put the collar on Jack at home to get him used to it. At first, he will probably try to take it off. Distract him with a game or treat, and he will soon stop noticing anything on his neck at all. Don't try to put the leash on at home as it'll attract the puppy's attention because it's new and he'll want to play with it.
Wear the leash outside, there are plenty of distractions and your Jack won't pay attention to it. For walks outside, buy a 5-meter ribbon leash.
A short one-meter leash won't be enough for the curious Jack, because he's an explorer, he definitely needs to stick his curious nose in every hole. On a short leash, he just will not get enough exercise. The leash should be a ribbon, not a rope leash, which can cut your hands if you jerk it hard.
At first, walk without letting Jack off the leash until you are sure he comes to you at the first call, "Come here!"
If the dog runs away from you, don't chase him. Instead, turn around and run away from him. He'll see this as a new game and will chase you. And when he runs up to you, don't scold him, but praise him and give him a treat, so that his approach to the owner is associated only with pleasure.
When he learns to obey you unconditionally, let him off the leash and walk on his own. During the walk, occasionally call Jack to you and put on the leash. Then let him go again. Don't put him on a leash just before you leave a walk, or he'll develop a mental connection: leash - a walk is over. And he'll try to run away from you to run around the yard some more.