So let me get this straight - this guy walked out on his wife and child because staying with them was preventing him from being able to go out and live his life however he wanted, and now he's just utterly baffled that his ex, who is now a single mom, might not be able to just go out and live her life however she wants? Did the kid just stop existing as soon as he was out of LW's line of sight?

Like, it's not clear from the letter what the child support situation is financially, but the way it's written definitely doesn't sound like he has much if any custody (or like he's particularly bothered by that) or does any meaningful amount of parenting. He goes on about how sad it is that she's moved back in with her parents, but they're probably doing more to help with the kid than he is.