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.