but then the most important thing would be if the boy - felt- he was female instead of having more-than-usual traits associated with females.
How does one 'feel female'?
Not meant aggressively - the whole schtick does sound initially convincing. It's when you start thinking it through step by step like this that you see the holes.
Trans women are women? Well, how are they women?
Not physically. Not even superficially physically in the vast majority of cases - well under 10% have genital surgery. (Can't say I blame them - genital surgery in either direction is horrific - very drastic, multi-stage procedures, hugely risky, massive complication and failure rate and will never result in anything fully functional.)
(Incidentally, neither surgery nor hormone treatment are needed to get a GRC; and about 80% don't have a GRC either.)
Not by virtue of their brains - there is no 'female brain' that exists separate from a female body (Gina Rippon's The Gendered Brain is a really good breakdown of the science of this - although I warn you it's one of those books that has you stopping every 2 sentences to add to your reading list or look up an interesting person or bit of history).
I don't believe in souls, so it can't be that.
What they wear? Well I'm currently wearing jeans and a T-shirt. Does that mean I'm not a woman, or that all men in jeans and T-shirts are?
How they act? Surely that's just stereotypes. Prancing about waving a handbag and screaming at mice doesn't make someone a woman, and I can't think of a single woman who does that anyway.
How they feel? How does a woman feel? What feelings do I have that I share with other women but not with men?
The harder you stretch for an answer that makes sense, the further one is from your grasp.
Eventually you end up concluding that the reason for that is there isn't an answer that makes sense.