Boku no Diamond Star

Takuma is a super ordinary first-year high school student in 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 idols live in concert. However, one day, Kei, his bias, suddenly suspends his idol activities due to health problems. Takuma spends his days in despair until Kei 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: Kadokawa, translated)