I do not know what your stepson is yelling and swearing at you about but I would practice "listening" to it out of curiosity, to see what it is all about; whether it is actually about something you have said or done or just "noise" from his pain.

I imagine your stepson does not have many words to express himself and is very frustrated about his situation and is sort of like a person with Tourette Syndrome with vocal tics, who cannot help himself very easily and just randomly swears (coprolalia). I would work to not take most of what your stepson says personally, not by avoiding him but by responding positively when you can (if he has a legitimate complaint or appears frustrated because he cannot do or express something) and "recognizing" him when he speaks. Looking at him when he speaks and saying things like "I am sorry you feel that way" said sincerely can probably go a long way to maybe calming him some or, asking if there is anything you can do to help (rather than automatically assuming you know what he wants or needs). If he says there is nothing you can do to help but "get the f*#$ out of my face/room" then saying, "I would appreciate it if you did not speak to me in that tone/fashion/way/with those words" and looking at him to make sure he hears you (and he may swear at you again, which means he heard) and then leaving, lets him know that speaking to you poorly will get you to leave. If you have to, leave in the middle of whatever the two of your are interacting about. Just say, "I do not wish to discuss this with you while you are swearing" and leave. That gives a clear message/boundary.

I am a little unclear if he has physically threatened you? That cannot be tolerated at any time, no matter what. If you have to, your safety comes first so call the police.

Other people yelling can feel very frightening but before I understood about anger I still realized that yelling is just yelling. Feeling frightened is uncomfortable but will not actually hurt me (unless there is an actual physical threat). If I can listen to what the other person is angry about, try to understand the other person and focus on them it is like learning one can understand a foreign language and instead of being so frightened, I begin to feel competent in navigating difficult situations.