Here is what I honestly don't understand... I see it being said that you are going to secret away the concept of your son's gender from him because you don't want to influence or confuse him. That argument is steeped in the idea that you CAN change or influence a person's gender identification (as you said it is not biological).
By indicating that your words (and for heaven's sake a shirt) can so easily influence a person into gender identification, you are making the argument for all of the crazies who try to influence their kiddos into *not* being a certain gender. By this logic, if my DS tells me later in life that he feels more feminine than male, all I need to do is change his wardrobe and buy him some football pads? Do you think that all of the dinosaur shirts or references to being a "man's man" could have made your DS's uncle into a man's man? My guess is no... so why do you think that simply calling your son a boy, dressing him in clothes that are generally for boys, and allowing him to follow his interests will turn him into anything other then what he is going to be?
see what i mean?
think about this- have you ever been in a situation where someone made you self conscious to the point you behaved in a different way than you normally would were you not being judged? for example, if you are a meat eater, did you order the salad because you were with vegetarian friends when you really wanted a burger? think about that feeling and multiply it by a thousand, and imagine how a child is going to react and how that child could possibly - for example- pursue his dream of dancing the ballet when "everyone knows" that ballet is for children identified as girls.
all the pink tshirts tell him so. all the models he sees on television tell him so. his friends and family members may have consciously or unconsciously steered him towards something else every time he tried to dance-- or if he did put on his sister's ballet shoes, when he went out in public, people made comments (and not necessarily discouraging ones) that made it clear to him he was doing something unusual and not socially normal.
it takes more strength and support than most people have to make defiant choices like that.