What did Grandpa do that made Billy feel proud in Chapter 15?

In chapter 15 of Where the Red Fern Grows, Billy, Papa, and Grandpa are on their way to the campground when they decide to stop for the night. While they’re waiting for supper to be ready, Grandpa suggests that Billy feed his dogs some corned-beef hash. Grandpa soon learns that Old Dan will only eat his food when Little Ann has been given her share. Grandpa is amazed by Billy’s dogs and comments that he’s “always felt like there was something strange about those dogs.” He notices that the dogs are always watching Billy.
As they’re preparing to eat, Grandpa does something that causes Billy to feel a sense of pride. As Grandpa pours the coffee, he pours a third cup for Billy. Billy feels like he is one of the men, noting that his head “swelled up as big as a number-four washtub.” He knows that Grandpa must also feel proud about how well Billy has done with his dogs.