I at once thought of Ciska, but after reading properly I think you meant
Francis for a boy?
It isn't too big a jump from
Francis to
Finn. I mean, if a person can be
Bessie short for
Elizabeth, then why not
Finn for
Francis? I've met someone who was called just
Finn though... at least, I think he was. His brother was
Mark.
You could also maybe call him
Francis Finn -lastname- and then if he's asked why he gets called
Finn, he can say "it's my middle name" instead of "it's short for
Francis" which might make some people be like, ".... really?"
Just thoughts :)