Items needed: a 15 to 20 foot leash, and a large ring choke style training collar. Each ring should be at least 1/2 inch in diameter. Since these can be hard to find, the next choice would be a soft material slip style collar.
Get your dog, and slip the collar over his head. Make sure you don’t let him flip the collar off. You can use some sort of clip or leather stop to make sure that doesn’t happen.
Next, take your dog out to a large area where there is open space and no other animals to distract him. Note: Do not go to an enclosed area that is distraction free, as what your purpose is is to make him pay attention when he loses focus on you.
Let the entire length of the long-leash out and let your dog go exploring. Meanwhile, start walking in a different direction.
When your dog gets to the end of the leash and is starting to pull that way, give a command of their name, loud enough so they can hear it, and immediately turn your body in another direction. Keep going until the dog again is distracted and goes off in his own direction. Again when he gets to the end of the lead, call his name loud enough so he can hear. Continue with this training until he starts really paying attention to you.
Very soon, he will just be focused on you, and follow you everywhere without correction.
After this training, any time he doesn’t come to you right away when called, repeat this training. Eventually you will have a very well trained dog who won’t leave your side.
After doing this, never take your dog outside or on a hike without a leash, as Jack Russells are above all, hunting dogs, where quarry is their PRIME MOTIVATION. They will almost inevitably take off after a squirrel or rabbit as that force if bred into their DNA.