He knews idols more than her, we don't know but from the flashback Sakura borrowed a cd and gave him it back after watching/listening it
So maybe he was that odd guy in the class hardcore idolfag and when Sakura got into it the same thing she asked him things like "what are the good groups?", "where to find events", etc. things like that and with time he fall in love with her, maybe even dreamt to become her manager and take her to the Top
And then that day came....