Wow, I can see that this must be a difficult situation for you.

So, therapy would be a really good idea. They do have to report you if they think you would possibly hurt someone else. Perhaps you could start off by describing the situation and your feelings and mentioning/admitting to only the verbal attacks you've made. Although even just verbal abuse can be damaging to a child, they are less likely to haul you away for it (unfortunately, in many instances).

I can tell that you know 100% that the verbal and physical stuff is wrong, and I know you want to stop, which is why I'm recommending omitting a few things so that you can get the help to make you stop.

A CBT (cognitive behavioral therapy) oriented therapist might be able to help you learn techniques to calm down and refocus yourself in the moment. It sounds like you already understand, when you're not overwhelmed, that your boyfriend's son has emotional trauma that is causing him to act this way. His "playing dumb" is likely a cry for nurturance and love. Not that it's your job to force yourself to give those, but ultimately his actions are very understandable.

Would it be possible for you to also do some part-time work or volunteer activities outside the house? Taking breaks from the kid could be really helpful.

I think you could also sit down with your boyfriend and tell him calmly that you want to do your best with his son, but there is more going on emotionally with him than you can handle. It might be that he should spend more time at home or taking his son out, rather than you doing it.