Why would it be wrong to express what he is feeling as long as he isn't being abusive about doing it? It's okay that our kids have concerns or issues with what happened growing up. Our oldest son has let us know things like this too, and yes, it makes us sad because we did the best we could. But the reality is that most parents do the best they know how to do and probably come up short somewhere along the line. Sounds like your son is a fine young man now despite that he feels some of his parenting may have been lacking along the way. Unless you left something out, it sounds like he's just being honest, not hateful. There's a difference, a huge difference.

Talk to him about the sadness you feel but try to affirm that it is okay for him to feel what he feels. All you can do is communicate with him now, acknowledge your part if needed. That was his experience, his perspective. While it might not quite match up with your own, it's okay.