It's very possible. But it's also very possible that the teacher is not doing it on purpose. When I was teaching high school, I occasionally had the student who I knew was way more advanced than the rest of the class. When looking at these students' papers, I always had the urge to hold them to a higher standard than the others, because I knew they were capable of more. I always had to go back and recheck my grading to make sure that I didn't take off more points from the person who I knew could do it easily, vs the kid who was really struggling in the class. Never intentional, but occasionally I caught myself and had to fix the grade before returning the papers.
I would definitely schedule a conference with the teacher and speak to him/her about why your son's grades have dropped. It is possible that this was vindictive, but most teachers don't do that.
