I haven't done Chiaki's ending, but I looked in the game code to see what the conditions are. Note: in cases where I don't specify the name of the memento, it's because I haven't obtained it in-game myself. The game code only specifies the index, not the name.
To get Shuuki to keep his promise to Chiaki, you must fulfill at least 3 of the following:
- Use Sumire's letter in active mode > pick option "Disagree"
- On Chiaki's path, there is a memento that lets you talk to him in active mode after he's visited his grandma. The game dialogue goes: "The two of them start walking back to the dorms. Chiaki is being very quiet, however, which defeats the whole purpose of Shuuki coming to see him. Maybe I should say something?" > pick option "Do you want to talk about it?"
- Use Pamphlet in active mode > pick option "You're so diligent."
- Chiaki has some sort of puzzle memento(?) that you can use in active mode > pick option "Is something bothering you?"
- Chapter 4 when cooking > pick option "I need help"