Where during Into the Wild does Ron Franz talk to Chris McCandless about adoption?
Chris meets Ron Franz in Chapter Six, when Franz gave him a ride back to a hobo camp. Franz took a liking to Chris, and they spent a lot of time together during the next month. When Chris left, Franz felt sad and lonely. Chris returned to California after failing to find work in Seattle, and Franz gave him a ride towards his next destination. While they drove, Franz felt that they were close enough friends to ask Chris for a favor:
"When I'm gone, my family will be finished, gone forever. So I asked Alex if I could adopt him, if he would be my grandson."
McCandless, uncomfortable with the request, dodged the question: "We'll talk about it when I get back from Alaska, Ron."
(Krakauer, Into the Wild, Amazon.com)
Part of Chris's journey was to free himself from all personal ties to civilization, and he felt that this request was an extra attachment that he didn't need; he'd taken steps to remain friendly with others but not to become part of their families, and while he wasn't cruel enough to refuse outright, he still made it a non-issue. Franz never saw Chris again, and the news of Chris's death hit him very hard; he adopted some of Chris's philosophies, and died a few years later after traveling the United States.
As stated above, it's in chapter 6 that Franz broaches the idea of adopting Chris. This is not a random idea on Franz's part. We learn that Franz had, in the past, informally "adopted" a total of fourteen Okinawan boys and girls, even sending the oldest to medical school.
When Chris arrives on the scene, this, in the words of Krakauer, "kindled" Franz's "long dormant" parental instincts. This means it had been a long time since Franz had felt the desire to adopt someone, but that he did feel a strong sense of kinship with Chris.
After church one Sunday, Franz talks to Chris about getting an education so he can have a better life. Of course, as Chris informs Franz, he already has an education and lives as he does by choice. He, in turn, urges Franz to embrace a more daring, wandering life.
As Franz drives Chris north part of the way towards his destination, Alaska, he asks if he can adopt Chris as his grandson, telling him he was an only child and his only son is dead. He doesn't want his family line to be finished, and Chris could carry his family name forward.
Chris dodges the request, which Krakauer labels "claustrophobic." Chris, so far, has avoided attachments and he "slipped painlessly out of Ron Franz’s life as well."