I have two suggestions.

First you might want to leave the leash on him any time he is out and around the house or yard. The leash will snag on things and he'll get tangled on chair legs, shrubs, and other stuff. This gets him used to the feeling of a leash and the pressure of the collar on his neck. Also, when he gets hooked up on something and can't go where he wants, he learns that its no use fighting the leash because he won't get anywhere. Be sure you only do this when he's supervised and don't leave him in the cage with his leash on. You don't want him to choke.

The other thing I would suggest is the Gentle Leader. It's a head harness for dogs, like a halter for horses. A strap goes around their snout and you hook the leash under his chin. Any time he pulls, his head goes down, which is a submissive posission. The harder he pulls, the more his head goes into a submissive position. The dog learns really fast that pulling is going to lead to being put in that position. Works great for an alpha dog. My dog was a real handful and this worked like a charm on her. At about a year, she doesn't pull much using her regular collar.

Good luck.