In the novel, Walter makes his son drink in order to teach him a lesson. More specifically, he wants his son to know that alcohol can be dangerous and that it is important to be careful when consuming it. In addition, Walter likely also wanted his son to experience the negative effects of alcohol firsthand so that he would be less likely to drink excessively in the future. Overall, then, Walter's decision to make his son drink was motivated by a desire to protect him from harm and help him learn about responsible alcohol consumption.