How can I prevent a boy from being "feminized" among her sisters, mother, and grandmother?

First things first. If you don't stop using the female personal pronoun to refer to him, he won't stand a chance of turning out normal!

I am afraid he will become somehow "feminine" due to a lack of male role at home. … Father is rarely home due to his work.

Don't you realize that you have answered your own question? Seeing Dad work to support the family, even at the cost of being deprived of the cherished company of his wife and children, is the best example imaginable of what one important component of being a true man consists in.

As he grows he will naturally be more attached to Dad than you--don't get jealous and try to hang onto him.  Don't interfere with their relationship.  Let your husband be in charge of that relationship. Don't try to prevent your husband from disciplining the boy when he needs it.

Don't force your son to be your confidante  or buddy because your husband is gone and you are lonely.  

Don't spoil him because he's your "only son".  Pampering boys leads to effeminacy.  Don't go out of your way to celebrate his birthday or his achievements.  He needs to learn from childhood that exceptional behavior is expected of him and not considered special.  Keep it low key.

If you can arrange it try to put him in an all-boy Catholic school.  Or hire a man to tutor him.  Boys don't do as well with mom as teacher as they do with a male.

BTW, I have many sons and no daughters.  These are some of the things I've learned over the years.

 :dancing:Be the kind of woman you'd like your son to marry some day.

Make him wash the dishes, then he definitely will not want to be a girl.

By the way, good for you for homeschooling. If you can do teach your kids just as well if not better then public or Catholic school then why give your kids to strangers for 8 hours a day ?

Don't worry about the peer thing either. My sisters two youngest ended up being junkies and in jail because of their wonderful "growth" experience with peers in the public school system.

Don't allow the grandmother to watch him or tell him what to do.

Darn right. She might let him watch Dumbo or Pinocchio or encourage him to read a dirty book—something diabolical like The 500 Hats of Bartholomew Cubbins.

Oh the humanity!