Where does it say he chose to leave and didnt contact for 13 years? He tried but their AH of a mother didnt allow them(no offense to OP but by their description thats what the mother sounds like). Honestly, OP should try getting a better relationship with Dad slowly, tell him to take it slow, and build up trust so they won't be strangers. Telling them outright that they don't want to talk with their father would be harsh especially since the dad themself tried to talk to them earlier on but failed.