Of course he is Jewish. Most of our generation are tinokim shenishba, but this doesn’t change who or what they are.
Mark is a very good person, and from mutual contacts, I understand that he is very sincere in his day to day life. He supports hospitals, education, and when he discovered animal cruelty issues, started only eating meat that he has personally purchased and slaughtered.
Over the last couple of years, Mark has started to get more in touch with his Jewish heritage, and we should encourage and welcome this step. I daven for his continued progress and success.