Help!

Okay first of all - if you haven’t already - some ground rules should be put down that he cannot mess with his sister’s clothing, as that is violating her personal space. He might not fully understand that what he was doing was invasive.

Secondly, what your son may be going through could be gender dysphoria: https://www.autism.org.uk/advice-and-guidance/topics/identity/autism-and-gender-identity

Now here’s where things become my personal opinion. If I were in your shoes, I’d probably make it clear to him that it’s okay to try on women’s clothing, just not someone else’s without permission. However, your personal views may differ from that.

I've told him that he shouldn't be messing with her things without asking, and we have told him that it's ok to feel however he's feeling and that if he wants to do that then it's okay we're here to support him, I just wish I understood his feelings and he was more expressive 

Awesome, I’m glad you already did so much! Yeah, I imagine he’s probably pretty embarrassed about the whole situation, so that would make it really hard for him to open up about it. From my point of view, I think you’ve done everything you should do. At this point the best thing might be to wait a while and see if he opens up about it on his own. I agree that if you press too much, it might push him away further.

Awesome, I’m glad you already did so much! Yeah, I imagine he’s probably pretty embarrassed about the whole situation, so that would make it really hard for him to open up about it. From my point of view, I think you’ve done everything you should do. At this point the best thing might be to wait a while and see if he opens up about it on his own. I agree that if you press too much, it might push him away further.