Re: kamae problem
Although I agree that those who come to class should make an effort to do what they are shown by the teacher--
this guy is could be really valuable for you! I love training with people who do things "differently." Since you know full well that you can at any moment kick him in the groin, you don't have to worry about anything (he does, not you!). So you get the chance to see if you can take balance, etc as you expect, even though this guy uses a different body shape than others.
Since he acts not as he is told, you get to see if your stuff works or if everyone else is just going along with you.
Also, when you are uke, does his stance choice allow you to regain balance more easily than you are used to? All great things to explore with him.