Knowing that your partner is doing something wrong is easy.
Knowing what he (or she) is doing wrong... that is something completely else. Learning to analyse this is a part of becoming a teacher. And the ability to perform this analysis often is the difference between a mediocre teacher and a really good one.
So I'm echoing Mary Malmros here. Don't tell him what to do. You are probably showing bad form in doing so. IF you've understood his problem you can help him without words. If you haven't, you can't - then leave it to the person teaching the class. Or silently by yourself practise your ability to analyse technical faults in others, then non-verbally guiding him into the correct form and see if it works.
If he's having problems doing the technique on you, he probably should ask the teacher to come over. If he doesn't call the teacher, for one reason or another, as Walter suggested you could do it for him.
Thank you, Hanna! I appreciate the advice. I don't really feel bad about trying to help him, ONLY because my sensei has told us to help each other and give feedback. Otherwise, I wouldn't do it. I'm going to have to try it non-verbally, though, as you suggested. Thanks!