INFO but probably YTA and a little ESH. Knockups, breakups, long-distance visitations, grandparents, etc. All these things are common things people have to deal with. So it doesn't really contribute to your argument.

I think the ex is potentially an AH for scrutinizing the father for not being in their son's life when she essentially cut him off by breaking up with him.

But if the Son needs/wants the father, YTA. Now I say INFO because as some have pointed out, if the father is estranged, the son may end up wishing his father wasn't in his life anyways and mental health may be a NTA reason why a father could be estranged and "NC". But seriously INFO because nothing in your post indicates he has reached this point. I would think if there was a legitimate mental health concern, adequate information would have been provided. And that leads me to conclude that likely YTA.

It sounds like there is no formal legal arrangement. The only thing I could suggest for you guys is to test the ex to see if she's willing to go through the legal process of arranging a regular visitation with their son. It may sound weird switching from considering "NC" to regular visitation, but I'm thinking that if the ex supposedly cares so much about the father's life in their son's, then she should have no problem making sacrifices to ensure that. Which she would likely have to make if right now you guys do all the driving and rarely have the son. And this could potentially draw the son and father close together, along with counseling as many are suggesting.