I read the book. The movies depart from the book in many instances, and in terms of the movie series, the MOVIE is considered "canon" as opposed to the book.

The book goes into a long subplot about how Lucy Mancini moved off to Las Vegas and hooked up with whatshisname the doctor dude, and clearly in terms of GF III, that portion of the book has been completely contradicted.

The movie plays it very vague in terms of how long Michael was in Sicily but my impression was it was only about a year. And of course the movies, all of them, give no hint whatsoever that Michael had a son by Apollonia.

FFC made it a point in GF II that Michael wanted to have a son, an heir. Michael tells Kay that it's important that they have chidren ... "our children." If he already had a son in Italy he wouldn't be that obsessed by it.

Of course, if they made a GF IV, they could do anything they want, but manufacturing a lost Michael son in Sicily would be extremely hokey storytelling. For me it would be much more believable to have Anthony, consumed with rage after Mary's killing, suddenly deciding to reverse his field and get into the family business. (Or, for that matter, there are the "legitimate" sons of Sonny ... Frank and Santino Jr. You could bring them into the plot as well and at least you wouldn't just be completely making up a new character.)

That way you could have the dramatic angle of Michael's son entering the business without having to go to the Bizarro-World dramatic absurdity of making up a son nobody had ever heard of for 40 years.