But
he happens to be a
German called
Florian.
I don't think it works on its own. Some nicknames do but
Flo isn't one of them. It's too short and sounds like a informal nickname, like Chlo,
Jay,
Bee or something rather than something more substantial like
Katie or
Mike.
I love both
Florence and
Flora but like
Flora best.
Have you thought about
Floriana,
Florinda,
Floella, Floria or Florencia?