son-out-law

noun2

1. •
   A live-in boyfriend of one's child.

   • “You also treated Haydar like your own son,” Didace observed. “He was my son-out-law, remember,” Henock prompted. — 2013
2. •
   The ex-husband of one's child.

   • The parents decided to give their daughter and “son-out-law” the cold shoulder. — 2014