sorry to harp on, OP, and thank you for this thread because lots of what you have said is very enlightening. You mentioned growing up in the 70s, and i definitely think there was a lot less gendered clothes/toys nonsense then.
Are you a feminist?
I ask because you haven't, for me, really answered the question about why you regard some things as "boy stuff".
He was always doing boy stuff from an early age and was a Taekwondo black belt at age 14. I have never been very feminine though and both my children grew up with a wide range of toys and activities of all kinds. As a family we did lots of camping, walking and cycling.
This is what confuses me in all the discussions about trans issues. I did (and do) most of these things, as did my family (mum, dad, brother, me) and i have never felt that i was not a girl. Having said that i cannot really put my finger on why i feel that I'm a woman, and frankly think that "gender" should be something we ditch asap (so that we don't say "dresses are feminine" "power tools are masculine" etc)
Do you think that your family attitude that these things are masculine types of thing may have pushed what in my day would have been called a tomboy (urgh - i got a lot of that even though i sewed my own clothes. Mostly dresses) into thinking that he wanted to be a different gender to that assigned at birth?
There's an interesting thing elswehere about what is actually required when you have to live for 2 years in your chosen gender (in this case as a woman) and the answer was basically using pronouns and changing your name all over the place and gender markers where possible.
I find it very hard to reconcile "this is boy/girl stuff" and no requirement on the part of authorities for anyone to indulge only in boy stuff if they are a transboy/man.
From your posts i glean it is really more of a deep-feeling of being male, despite your insistence on activities and clothes being gendered things.