Whatever issue that guy had could have been resolved later through other channels.
Walk back to the curb, call the police, and let them resolve the issue. If the ex-husband had a legal right to his son then that means the ex-wife and her BF had no right to keep the son from him. So you potentially have a man protecting his property while simultaneously preventing another man from his child. The father shouldn't have been aggressive but at the same time you also can't kill someone who is just trying to get what is rightfully theirs.
Imagine if someone were keeping your kid from you.
This happens every day in America -- a parent somewhere keeps their child when visiting hours/days are over. Even if the other parent consents, it's not legal. A court order mandates when the child will be with which parent. If either one deviates, they are potentially in contempt of court unless they clear it in advance with their case worker. Otherwise, it would be constant accusations of "He kept Sam an extra day without telling me;" "She agreed to it;" "No, I did not!" It could easily be a way of getting the other parent in trouble with the court, possibly as a ploy to gain full custody.
If a parent refuses to give the child to the parent that has legal custody, that can become kidnapping, which involves the feds if they take the child out of state.
You can't separate trespassing from "someone who is just trying to get what is rightfully theirs." Unless the child is in danger, you wait for the Cops, and then maybe the courts. Breaking a trespassing law, and maybe disturbing the peace or terroristic threatening, to try and enforce a visitation order is poor optics. You'll be looked at as a bad role model for your child -- using force rather than peaceable means. And, if arrested and convicted, you'll be on the path to "unfit parent."
Everyone has a cell phone that records voices, images and video. Just use it to record actual events and statements, and you'll have law enforcement and child welfare workers on your side.
Nothing that happened before, during or after the shooting can possibly be good for the child. That's the bottom line.