Why does Gene agree to Finny’s request that he not live by the rules?

From the text, we can infer that he was placating Finny, going along with him because there was nothing else to do but agree.

“I think I’d better get to the station. I’m already a day late at Devon.”

“You aren’t going to start living by the rules, are you?

I grinned at him. “Oh no, I wouldn’t do that,” and that was the most false thing, the biggest lie of all.