@jokerxhisoka he said he wouldn't pick what she was going to pick, with the implication of "I wouldn't want to take what you want at the end", or something to that effect
What Ishikawa said in the last panel is
>...then
>I WOULD be eating Katsu curry....I guess.
The implication is he wants to stay with Yamada during the end of the world.
The correct answer is to go out on a binge eating all the couple's menus in restaurants out there. It's totally not a date, just the two of you enjoying your last day to the fullest.