Both you and your son seem to have trouble with objectivity where this teacher is concerned. Please don't take that as a criticism or insult, just an observation from what you've said. I promise my intent is NOT to scold in any way.
You mentioned you decided you didn't like her the first time you ever met her and even before you knew she was going to be your son's teacher, but on the other hand, you said when you actually sat down and talked to her she seemed okay. You mentioned other parents referring to her as strict or stern, but you realize your son may be "reading" her as mean and critical because he is pretty sensitive. You say you are angry at this teacher and feel she is unsympathetic and that you are therefore unsympathetic to her which means you have your barriers WAY up.
You may not realize it, but you appear to have turned this into a personality conflict rather that this really being about your son's academics. She called my son a "weirdo" (which is hearsay and probably very much out of context), so I'm mad at her. By the way, as teachers, quite honestly, yes, we do have students that we may describe as weird, unusual, etc., but that doesn't mean we like them any less, are unable to help them, etc. Some kids are a bit out of the norm. That's normal

. We're used to it. And it is okay if she sees that in your son really. Some of my greatest affections are for my "weirdest" students.
Rather than get caught up in how you "feel" about this teacher, perhaps focus WITH her on how best to help him in math. Should he be in tutoring before or after school? Some schools have pretty regular tutoring sessions available, and sometimes the teachers kind of divide them up: one teacher does writing, one does math, etc. Is there a set up at their school along that line? Could another teacher perhaps work with him in a tutoring capacity on math? Sometimes hearing skills from a different teacher's mouth is just what a student needs as we all teach slightly differently even over the same concepts.
If something gets graded incorrectly, rather than seeing it as an attack on your son (which is how you seem to take it -- and how your son seems to take it also -- you might note that by the way), find a way to communicate about the error matter-of-factly without coming off as "gotcha" or "you DID this to my son".
And I know you say your son won't advocate for himself, but it is a skill he really, really needs to learn. If you have to, go WITH him and have HIM talk to her about the question, but duct tape your mouth closed

. He'll have you as a support there if needed, but he will also learn that he is capable of asking simple questions about his work and the teacher won't grow horns and eat him alive if he does ask questions.
It just keeps feeling like preconceived notions and misperceptions and misinterpretations and mind-reading and all of those unhealthy communication skills that as adults we know we have to get past. Your son is headed down the road to those same poor skills, but he is at a ripe age to learn new communication skills. You and the teacher are going to have to model those skills for him. In fact, it might be a good idea to really talk to the teacher about how your son perceives her and his fears about asking questions, etc. Approach it as informatory, not accusatory. Then she could really work with him directly to encourage him to ask for help, etc.
It is not unusual for teachers to see these kinds of misperceptions and fears and behaviors, but sometimes we don't realize that is what is going on unless a parent really has a talk with us from the perspective of how can we as a parent/teacher/student team work together to overcome these fears and get the focus back on the academics.