Papa's Assasin

An assassin is retiring after he finishes dodging bullets from his current assignment, all while fantasizing about his future leisurely life, when a strange woman throws a little boy down to him and begs him to, "Save this child!" 14 years later, instead of managing a fancy art gallery he's doing manual labor and raising an increasingly distant teenage boy but he's happy to provide a happy home for his new son until he realizes why Ryuunosuke has been acting so strangely....

(Source: Dangerous Pleasure)