So, from the pov of someone whose son wore a skirt to primary from about 5 & loved dresses that he wore often (though I went with what I'd buy a daughter so nothing frilly or excessively pink etc) who then, around your sons age, declared (hysterically & out of the blue) that he was trans...
This against a background of total acceptance of 'clothing rights' - ie, you can wear whatever you like cos they are just clothes, they don't mean anything. We prepared him for the comments he would get & he was super confident & unbothered. Often his reply would be along the lines of 'but it's ok for a girl to wear trousers?'
It took some unpicking but essentially, he was being coached into the idea by his wellmeaning friends cos the idea was already there that a boy in a skirt = transgirl. We chatted & talked about the need to get to know his body as it is & we'd see what happens. That he was to young to even know himself as he was before wanting to change it.
He continued with his clothing choices though I drew the line at a skirt for secondary on the basis I'd also refuse a skirt for a daughter (on the grounds of it limits movement & makes you to aware of how your sitting, walking etc etc etc) (and no fucking pockets!).
Anyway! He's just a plain ole gay boy. It's that simple. Other people put the idea onto him cos it's just so unacceptable, ultimately, that a boy could lower himself to dress 'like a girl' without there being mental illness involved.
Where we are now - he's 'openly' gay (stupid phrase but I mean he's not ashamed & hasn't hidden it or anything). Mostly grown out of the wearing dresses etc (probably to do with being 6ft 1 & too tall for all my dresses & shoes). We've spoken about what happened a couple of times & he laughs at the idea that's what he thought & is thankful for how I dealt with it. Ie - I didn't just start him on a pathway...
We're also about to get him assessed, I think, cos it's becoming increasingly obvs he's not wired in the same way most others are, at all.
So yeah! Let him wear what he wants & impress upon him that it's actually fine for a man to do that, it doesn't change who he is, it's just pieces of fabric. There's a much higher chance that he's gay than trans & likely higher again that he's having confused thoughts & isn't sure how to process them due to his ASD which makes him far more vulnerable to ideas that might not apply to him.
If you wouldn't let a daughter his age have extensions, then don't let him. I certainly wouldn't. Things like make-up & nails are fine too, just not for school & not to 'beautify' & sculpt, but to be creative. Again, the same rules as you would for a daughter.
To be brutally honest - my son (nearly 15) has become incredibly sceptical of the whole movement now after his experience (which happened thru kindness!) & watching what's happening around him. That's not to say his doesn't believe in gender dysphoria, just that young ppl are getting swept up in it).
I have tried to present everything as neutrally as possible so he (& his brothers) find their own way rather than me coach them.