I’m Not Your Steppin’ Stone (Shameless) (2001)

Blurb:
Sakai Kazuya is a 23-year-old man in love. Unfortunately, the object of his affection is a lovely woman only interested in well educated men. Sakai never finished high school and has been living off of his brawn, not his brains. So he enlists the help of her younger brother, Ezumi Masashi, to help him pass the high school entrance exam, his first step toward to becoming a well-educated man. Ezumi is happy to help, but his services come with a very steep fee. And, Ezumi isn’t interested in money, he has his sights on some of Sakai’s other assets…

Source: www.anisearch.com/manga/1609