I don't know about this. Unless you can tell with a glance that the boy is develomentally disabled (and even then), you are likely to get stares - if not comments, or worse - from others at the restroom.
Even though everyone has a different idea of the maximum age that is permissible for a boy to be and still visit the women's restroom, pretty much everyone would agree that eight is really pushing that upper limit. I understand why, and am not in any way disparaging the OP - just observing typical societal norms.
Twelve is far beyond that. I would almost guarantee that a twelve year old boy - many of whom are at or near puberty - would be viewed VERY dimly, unless his condition is visibly obvious. And even then, what if someone approaches from behind?
I'd hate the OP to have to deal with heartache from those who don't understand the situation.
These do exist in WDW. Do take a peek at the DISabilities board, they can help you with specific info. That will certainly make your trip much more pleasant.