Ichizu Sugiru Kouhai ga Ore wo Akiramete Kuremasen

Mikage is a jobless graduate from an underperforming high school leading a boring life. A handsome stranger suddenly comes to visit him, hugging and saying "I've come to be your lover". Naturally, Mikage panics! The handsome stranger turns out to be Mikage's high school junior named Amatsuka. He says he's come because he's fulfilled the conditions Mikage set to become his boyfriend at Mikage's graduation ceremony. Puzzled by it all, Mikage is taken over by Amatsuka's enthusiastic approach which winds up with the two living together at Mikage's place?!

(Source: Frontier Works)