I haven't equated autism with being trans at all, and I haven't said anything about it being good or not. Please read exactly what I said without putting your own spin on it. EDIT: Oh, and in this interests of accuracy, you know what your autism feels like. Not his.
The point is that life is hard for him, he sees people treating girls nicer than he's being treated, and he's drawn the conclusion that life is easier that way, so why not be one? It's nothing to do with being "set straight" on gender politics, it's about him and his experience. So, if OP lets him wear girls' clothes, one of five things will happen:
1 - He discovers that life isn't actually easier when dressing as a girl.
2 - He doesn't like it and stops.
3 - It's one of the temporary fixations OP mentions, and it just naturally goes away as he loses interest.
4 - He is actually trans, and he sticks with it.
5 - Turns out he just likes girls' clothes, and wears them occasionally.
There are few downsides, really - OP needs more information on what's going on in his head, and this is the only reasonable way to get it without external influence.
And yes, I'm diagnosed autistic. I'm making no judgement at all here, on the basis that I know a number of trans autistic people and the misdiagnosis rate is around 50% amongst them.