While it sounds like your step-son has other issues, my advice regarding the teacher is if a child is unhappy in class with a particular teacher, so much so that it's affecting his grades in that class, I personally would switch classes if given the opportunity. I know that it's the teacher/instructor that makes the class great, average or subpar and it's a crapshoot which teacher your child will get each year. That's at any grade level.
Remember the difference between the teachers you loved in school and those you hated? I learned so much more when I was in a class I loved than when I was in one that I couldn't hardly stand. I was very disappointed with my daughter's 2nd grade teacher, but her first grade teacher was amazing. I'm not sure what to think about her 3rd grade teacher yet, but if she's anything like last year's, I won't hesitate to ask the principal to move her. I think having that teacher put her back several months from where she was when she left first grade.
Elementary school is not like high school and college. In my opinion, the teachers have a greater duty to make it fun to learn. Once you get into high school, it becomes your responsibility to learn so you can go out into the real world when you graduate and get a job or go to college, but until then, there has to be some real effort on the part of the teacher to get thru to the kids or learning turns into something negative, not positive.
All that being said, with your step-son's other issues, it may not only be the teacher, but again, if you find talking with the teacher doesn't help the situation, don't completely exclude the possibility of switching classes. That may be all he needs to ramp up again and be the good student he's meant to be. Good luck!