How can my friend improve?

I’m trying to help my friend improve his riding but I’m a bit stumped. We’ve been riding for about 3 seasons and have 50-60 days clocked. He’s made decent progress but I still think there’s something missing and I can’t figure it out. My best description would be that the body position is very tight and rigid. I assumed this would just resolve itself over time and telling him to try to be loose but overall his body position now is the same as it was week 1, 3 years ago. For example, the angle of knee flexion doesn’t seem to change much when he’s riding. Is this an issue? Maybe it’s something else? If there’s any drills or tips you guys have, any help would be greatly appreciated.