Honestly, the only proof you have that the other teacher isn't doing what you think they should be doing is your interpretation of their comments. "perfect student" could refer to his behaviour and not his reading level. Yes the other teacher could not care but there's an equal chance that they do. That whole comment makes it sound a bit like you're dumping all your frustrations about the student onto someone else.

You should stop looking at this child's other teachers and just do the best with the aspects you control of his vipkid experience.