Here, the Master says that he was. But I can never tell when people are kidding.
So was he, or wasn’t he? Who’s got the Straight Dope?
No. In many of his songs he begins singing normally, only to break into his signature high-ptiched wailing.
He sang in a falsetto voice. It’s not very difficult, and rarely involves castration.
Cecil was kidding. Valli possessed an accomplished falsetto voice, but his natual tenor range can be heard on songs such as “Grease”, and “Oh What a Night”.
I was never much of a Frankie Valli fan, but my uncle and stepfather are. They went to see him in concert about 3 years ago. They told me that, VERY early in the show, Frankie told the crowd (cheerfully but firmly), “Guys, I’m gonna level with you- I’m not 19 years old any more, I can’t sing soprano any more, and I’m not even gonna try. But if you’ll bear with me, we’ll have a good time anyway.”
According to my uncle and stepfather, the crowd enthusiastically applauded his candor, Valli sang with his natural (NON-falsetto) voice for the whole set, and pretty much everybody loved it.
Since this is a comment about one of Cecil’s columns, I’ll move this thread to Comments on Cecil’s columns.
Cecil forgot (yeah, right) to mention that once you’re all grown up, it’s too late to achieve a falsetto voice via castration. Testosterone has already lengthened you vocal chords.
Prostitution, though, may be an option. There are a lot people out there with, shall we say, different tastes. 
Peace,
mangeorge