In the book Wonder, when does Amos first show up?

RJ Palacio first introduces Amos on page 122, which is when August's friend Summer arrives at the Halloween party being held by Savanna, who is one of the most popular fifth graders at Beecher Prep.
Summer expresses her surprise at how many couples are at the party, and Amos and his girlfriend Ellie are one of them. This is all we are told about Amos at this stage. We don't hear anything more about what he and Ellie got up to at the party, because Summer became very uncomfortable soon after realizing that Amos was there. Her discomfort was caused by Savanna telling her that Julian (the class bully, who constantly picked on August) thought she was cute and wanted to ask her out on a date.
Summer went and hid in the bathroom, and called her mother to come and fetch her.
Later in the book, we find out that Amos was initially one of Julian's close friends. He was one of the kids who later became disillusioned with Julian's cruelty and ends up later defending August and his friend Jack.

Amos appears for the first time in the chapter "The Halloween Party," told from Summer's point of view. On page 122, Summer speaks about all the couples at Savanna's party, including Ellie and Amos. The character of Amos next appears in the chapter "Back from Winter Break," told from Jack's point of view. Jack describes Amos as "a pretty straight-up kid" (page 168). The fact that even Amos only gives Jack a half nod means that Jack has really become a social outcast because Julian is punishing Jack for his friendship with Auggie. Later, Amos is mentioned as on Julian's side, in the list that Charlotte makes (page 177). Over time, however, Amos is one of the kids who becomes tired of Julian's nastiness. When Julian grabs Jack's backpack and puts pencil sharpener shavings into it, Amos grabs the backpack and hands it to Jack (page 209). Amos grows weary of watching Julian be so cruel, and he realizes that he should treat Auggie well.