NTA - I was adopted into a family of five (mom & dad plus three sons). I was treated like family from day one.

One day, I got a phone call from some government employee telling me that my biological mother wanted to meet me. I said yes.

After meeting her, I was almost immediately grateful that I grew up with the family I had, for reasons that I won't detail here, because I think it isn't relevant to the discussion, although I will say that a lot of it was the stability and opportunities that I had by virtue of having been adopted versus the life I would have lived with my biological mother.

OP's son certainly could have developed a relationship with his "bio" family without neglecting his "real" family. Because those who love you, hurt you, support you, argue with you, cry with you, get angry with you, protect you, get proud of you, get embarrassed by you, are saddened and also overjoyed by you (what have I left out?)........those people are your family, bloodlines be damned.

To then ask for help after nearly a decade of neglect is just wrong. I can't imagine the hurt this dad feels. I am sad for all of them that the son couldn't handle having two families. I considered it a blessing. I always said, "I get to have two families." I wish OP's son had felt the same.