Fern for Ferdinand is a bit gutsy, but I think it works. After all, if Fern turns 4 or 5 and doesn't like it, he can just say so and change his nickname.
Other options;
Finn
Findy
Feddy
Andy
Three boys nappin', one mama namenerdin'
To peruse my top 100ish names:
http://www.behindthename.com/pnl/82589
He is becoming greater, I am becoming less
Bits of the journey here:
http://jodifriesen.blogspot.ca/And some family cuteness here:
https://www.flickr.com/photos/41705228@N08/