Your child is going through a trauma. It seems the step mother is mistreating him. In as much as he is you, there is always truth in whatever he says. I don’t think he can come and complain to you for no reason. Give him a keen ear. Listen him out. He might be suffering from the inside. Now that it is even affecting him academically. He is going through mental torture. Sit him down and listen to his part of the story. There might be something that is going on. Especially when he is at his fathers place. Investigate what it is. If it is serious then he should stop going there for a while. Or you can have a discussion with your ex husband. I know both of you expect the best for him. The situation might go to worse. If you really care do something now. You might be late at the end run.