I'm going to give someone an ultimatum to identify himself online before I do it for him
Does he have a good reason for not wanting other people to know who he is?
He probably thinks his anonymity is a good thing for him.
I think you should leave the guy be.  Doxxing people online is wrong, especially if they're actively trying not to be identified.  It could hurt him and get you into legal trouble as well.
I'm not doing anything illegal, he has all the right to know who I am. He'll wish he never hid his identity.
That doesnt make a whole lot of sense. People have the right to remain anonymous online if they wish to. If you want to know who the guy is so bad than maybe you can just see if he would be comfortable only telling you.