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"
As you can see, most of the flags are based on mementos, some from fairly early on in the game. You may need to access an earlier save file to fulfill the conditions, depending on how you were playing up until now.