There is no "human" Danse. Danse was a Synth designed by the Institute, and most likely had his memories wiped (so he had no idea that he was actually a synth). What Namikaze's brought up in the comments is possible but so far nothing canonical has been addressed in game.
Paladin Danse has the key to the Cambridge Police Station. Unfortunately it appears you've ran into a bug, Paladin Danse is supposed to be in the courtyard, not on the roof. If you're playing on a console version of the game you will need to reload the game and hope that the next time you arrive at the Cambridge Police Station to provide Fire Support that he has spawned in the correct place.
If you're playing on PC you can use the cheat for noclipping and float up to the roof to speak to him. This should "fix" the issue (since he turns around and walks into the police station).
During my playthrough, when I sent Paladin Danse to Boston Airport he ended up behind a quest locked door that isn't opened until the beginning of the Brotherhood of Steel mission Liberty Reprimed. Unless you're willing/able to use console commands, he can't be reacquired until you start the mission and the door opens.
Obviously, the preventative workaround is to never send him to Boston Airport - use the Prydwyn (or any other location, really), instead.
Best Answer
There is no "human" Danse. Danse was a Synth designed by the Institute, and most likely had his memories wiped (so he had no idea that he was actually a synth). What Namikaze's brought up in the comments is possible but so far nothing canonical has been addressed in game.