Boku no Diamond Star

Takuma is an ordinary first-year high school boy from the countryside who happens to be a (male) idol otaku. He spends all his free time working part-time in hopes of one day seeing his favorite idol live in concert. However, one day, his favorite idol Kei suddenly suspends all public appearances due to "health problems." Takuma is devastated by the news, until one day when Kei himself moves into the house next door…

“I want to help. If there’s anything I can do, please let me know.”

“There’s nothing you can do.”

(Source: Pixiv Sylph, translated)