Ferdinand "
Fern"?! That is brill, I love it.
I guess you could also do
Andy / Nandy /
Nando / Dinny /
Dino or something. But
FERN, omg. I wouldn't change that. I mean, girls get to have boyish sounding nicknames all the time (
Charlie,
Alex,
Sam, etc.), I think it'd be cool to see a boy's nickname with a slightly feminine bend. It's not like it's an extremely girly-sounding name to begin with, though. I think it could be pulled off pretty easily.