I had this problem when I first met my DH. He used to try and reason with the BM and stay on the phone to her for ages explaining and pacifying her. I pointed out time and again that it did no good because she would just get worse and he really was at her beck and call. Eventually he began to see the truth in what I was saying, and started to be much more assertive, till it reached the point where he would put down the phone on her the minute she started raising her voice. Now he won't even talk to her, she has to put it all in texts or emails, and often then he ignores them if they are crap. If she shows up at the house she is not allowed over the threshold. If she were a reasonable person none of this would have been necessary, but she is paranoid, combative and in my opinion has a personality disorder.
You will just have to try and persuade your BF to get assertive, because this is the only thing that works. Hopefully sooner or later, he will, but there are plenty of step talkers whose DH's or BF's haven't and won't. More fool them.

Try reading some of the posts on this site that fit your situation. It helped us. FDH was really hard to talk to because I didn't want to over step the line. I got on here and read some of the posts and it was amazing on how so many people were going through the same thing. It also was easier than siting him down and saying every thing I have noticed. I stared with listen to this and chose a couple crazy ones that were way worse than the BM we are dealing with then I read a few that were almost to a T on what was going on in our life. It was really amazing how he had the light bulb go off right away and said how did they deal with it and he worked on changing things right away. I understand about being new and not knowing how to address your concerns. the best thing you can do is talk to him and this is a great way of getting that started.