Honestly, I'm not sure the LW has the right (morally; legally is a morass that depends on the various countries) to just write this guy out of the kid's life. Even if he decides to be an asshole and do nothing for the kid, there's going to come a day when when the kid is going to get to make their own decisions, and may very well decide to track Dad down (to say nothing of the possibilities raised by DNA-matching services). Whatever LW owes their ex, they owe it to the kid to tell their ex about it.