just found a gem, "mother's friend", korean erotic movie from 2015, basically this guy moves to his friend's place and starts a relationship with his mom behind his back, lots o softcore sex scenes, the son catches them multiple times and peaks through the door without intervening, there is also mother's friend 2 and mother's friend 3, but I don't know if they're as good as the first one, each movie is a different story with different charcters though.
http://iporntv.net/download/41294465/m.-s-friend-2015-uncut