You're on the right track
I have a 16 year old son diagnosed with PDD-NOS (Pervasive Developmental Disorder - Not Otherwise Specified). He has hydrocephalus and has a shunt. He has communication problems too.
In your case, the only thing I would add is ask the question, "Is he stealing the clothing from you and your daughter because he wants to be closer to you or your daughter?" I am not a psychologist, but sometimes kids with these kinds of communication skills, things come out sideways.
I was wondering too if you had him choose what women's clothes he would like, would it add to his understanding that it's OK for him to do this. Again, this is just an idea. Sometimes doing things and not saying things works better. It does with my son.
ChrisP has really hit the nail on the head here, and seeking a professional that would understand these things would really help here.
I wish you all the best with this, I know how hard it is to have a child with communication problems. You are doing a good thing by coming here!
-Tracy
You sound like a good Mom!
Just a few thots from when I was a 15 y/o boy.
1. The fact that his sis has/is wearing the undies MAY BE a turn on for him! And her NOT liking it, may add "forbidden" excitement for him!
2. U r quite lucky he is ONLY into undies! Expect THAT to change, if he is in fact, going to become a lifelong CD.
3. Buy plain undies for your daughter, if she wants him not to touch them! Expect him to be attracted to, and wear, her SEXY ones!
4. This could be just a phase. I went thru several stages of sexual turn ons in my teens. Altho CDing wasn't one of them back then. If u don't make a big deal out of it, he may move on to something else, like girls, eventually.