My boxing team is pretty small, so I only have 1 partner who is the same size and similar ability. So we spar a lot. We tend to get used to each other's style and it can hinder progress a bit unless we constantly try new things. Otherwise, we're just doing the same things over and over again. Maybe you also need to try new things.
Use an intense visualization session to think about how to overcome his defense and then try to apply your tactics. Get a friend to take a video and analyze it.
Dump the mindset that he's better than you. Go with the mindset that today, i'm gonna surprise the hell out of my coach.
Let us know how it goes and good luck with your progress.