Does he go to the vets regularly? Be sure it is not medical. He could be wanting to stay out longer or he could be in pain from the walk. It's up to you to watch and be observant and see what you think. Experiment....see if he wants to go out immediately again, or does he lay down and want to rest???
He's telling you something and you have to learn to speak "dog" When we take our muts out to the park, they head for the car when they are ready to quit.