Hi, welcome and thanks for rescuing!
There is a very good chance that she was not abused, other than by having someone create dogs of bad temperament.
Regardless of the why she is giving lots of good and clear warnings that she is not comfortable with your son yet. And just like with us, you can't make someone like someone else - that takes time and trust.
I would join this group: shy-k9s : shy-k9s
and start reading the archives.
Have you contacted her former foster family for any input?
Does she crate?
Because this has potential for bite you need to proceed very carefully - using safe containment, crating and rotating - when your son goes out, she can have free run, he needs to call before he comes in, so she can be contained again. When he is home, she is tethered to you or crated.
If you have a clicker, that is something fun he can work with her using. Tossing her treats when she is showing appropriate behavior (that you will be able to shape because she will be tethered to you) feeding her, and other things will help to build that trust and relationship BUT - I would be focusing more on safe containment, not expecting too much from her and looking at this as a longer term project so keeping their interactions limited to keep them both safe.