Keep him on the long line, and do exactly what you do when he's on the shorter one. If he doesn't come back when you call him, give him a gentle tug to encourage him. Or turn around and start going in the other direction (run if you have to). Make sure to treat him when he gets to you. If he comes right away, treat and have a party! ("Atta boy, Toby", "Yay!", etc) Something else you can do is play games in the house or yard (we call them "Puppy Recalls") Randomly call his name and when he comes to you, give him a small treat/reward.