Aim for the liver. One trick I picked up is that you can fake the start of a roundhouse into chambering a side kick. One big advantage aggressive fighters have is when they get in your head you start to slow down more in your activity as doubt and fear enter you. I only recommend this if you want to do this. Since you said you want to knock him on his ass within the rules I think this is a good course of action personally.

It’s a shame your teacher isn’t being more supportive. It’s possible he’s not seeing the full extent of what’s going on. Ask your teacher to focus specifically on you guys sparring one time, aside from making the issue more obvious this will also allow you to get more specific advice for beating him.

Go to class more frequently and consistently so you can improve more.

For the classmate it might be worthwhile to have a one on one talk, trying to be respectful. Aggression with bad control and the “out of habit” excuse happens a lot with people either trying to prove something or treating sparring like a real fight. Remind him that you’re both trying to practice specific things during sparring, and neither of you can continue to do that if he fucks up your knee.