Frederico 3
Fred 5
Fredo 10
Freddie 98

I searched Geoff's online script: Frederico 0, Fred 0, Fredo 49, Freddy or Freddie 7.

I think that, in the script, in all but one instance "Freddy" or "Freddie" are spoken by non-blood: Kay, Clemenza, Johnny, and Moe. This is the only instance where blood calls him Freddie:

MOE: ...You goddamn guineas really make me laugh -- I do you a favor and take Freddie in when you're having a bad time, and then you try to push me out!

MICHAEL: Wait a minute -- you took Freddie in because the Corleone Family bankrolled your casino....

So the one time he called his brother "Freddie" is the time he wanted to take Moe's exact words and shove them up Moe's backside. It's a great rhetorical device.