He may not be used to exercise & not have the muscle tone he should. If he has been caged for a long time, he may not be able to do too much until you recondition his muscles. Go just a little bit further each time you take him out. Try to make walking fun, train him in basic obedience when you take him out for exercise. If you have found out just what it is that this dog really likes, end your walks with that treat. ball, chew, treat, play, he needs a reward to motivate him. Now he dreads it, try to make him like it, make it fun.