I care because I love training in this Dojo, and as I said before, I feel indebted to him so I'm concerned about him.
Sure. But if your teacher creates a situation for himself it is not your job to fix it for him. It just wouldn't work.
Isn't there an implication here that if you train with this guy to prepare his grading test, and if he then performs poorly, that will also reflect badly on you?
If that is the actual problem with the situation, you could tell your teacher "sure, I could train with him but since he seldom listens to what I say I'm not sure that would amount to much good".
What does he feel about training extra with you? If neither of you are happy about it, it sounds like... not a good situation.
(Or possibly your teacher feels it is a problem that you don't really get along well, and is arranging this extra training so that you the two of you will hopefully learn to cooperate?)