The voice of the "boy Pavarotti" was most likely Marni Nixon, who dubbed nearly all the female "legit" singing in that time period. I agree that it was definitely not Pavarotti. Boy sopranos do not have that kind of vibrato.
I don't think Pavarotti traveled to Hollywood when he was a child!  

Not only was the "boy singer" not Pavarotti, in my opinion it was a woman soprano's voice that was used.
The tone and quality of the voice are quite different from those of a boy treble.)