Q. What's the best way to train a dog to use a lead again?
A. It depends on how serious your issue is. If you need to start from scratch: Bring out the leash, place it on the ground. Click and treat your dog. Say his name, work on attention, click and treat for attention. Work with the cheese sticks, or with some chicken.. something stinky, soft, and high value. Allow him to sniff the leash, praise him, click, treat, click, treat. Pick up the leash, click treat him. Hook the leash to his collar and allow the leash to drag, click treat him. Have him just follow you around, click and treat him to hold his attention.
Then, pick up the leash, click and treat him. Then drop the leash again, click and treat. Take baby steps. Then, hold the leash while you take a step, click and treat him for following. Open the front door, click and treat him. Then, take off the leash, click and treat him, and end training.
Pick training back up in an hour, and do the same exact thing from start to finish, only this time, "finish" will be you two going outside, you clicking and treating him a bunch, and then you bringing him back inside. Work your way up slowly. You can't expect to just bring him outside and bring him on a walk right away.
When outdoors, use a front hooking harness like the Sensible/Sensation harnesses: http://www.softouchconcepts.com/index.php/product-53/sense-ible-harness / http://www.softouchconcepts.com/index.php/product-53/sense-ation-harness. These harnesses will eliminate the pulling power of your pup in a positive way. This will put you in control without the use of force. Carry high value treats with you everywhere, and offer them for good walking behavior - treats like white meat chicken, cooked fish, turkey pepperoni, turkey bacon, diced ham, mozzarella cheese sticks, hotdogs, all cut into tiny little pieces. The more you work on walking on-leash/attention indoors, the better it will be outdoors, remember that.