Well, I don't know, but if this guy really has practiced for a good while somewhere else, maybe he's actually doing things he was previously taught were desirable and practicing in a way that took him a long time to learn to do? Or simply that completely different things were seen as more fundamental in his dojo and more important to learn first, so the things he's spent his time focusing on are not what you guys consider 'the most important basics' (and likewise he may look at you guys and wonder why so many people seem lacking in what he was taught were 'basics'). It's possible that the 'good' people in your dojo would even have been corrected in his last dojo, who knows. So maybe frustrating and confusing to come somewhere else and find that people practice so differently. Of course if he's going to train at your dojo he'll need to adapt, but it makes some of the confusion and frustration understandable, especially if he perceives you guys as insulting things he's actually proud of and feels he's worked hard to achieve.
Can you try to emphasize less that he's doing it 'wrong' and more that in your style it's done differently? Sympathize with him about how hard it can be to pick up a new style? Compliment him on any areas where he's strong?
It just seems that right now everything is becoming a battle which may make any kind of compromise on his side feel like 'giving in' or losing.
Though his comment about it not being a problem if Sensei doesn't say it is is actually pretty reasonable. DOES Sensei see it as a problem and want him to learn to practice differently? If so, he may not believe that unless he actually hears if from Sensei. If you guys are all saying something's a problem but Sensei seems (at least as far as this guy can tell) to be content, it's not surprising he might think you guys are overstepping your bounds telling him what to do.
Believe me if you watched or practiced with this guy you would understand our situation better. Our Sensei acknowledges this guy has no fundamental grasp of basic movement and thinks he the student is too "good" to relearn the basics. Sensei does want him to learn our style better but understands this guy has issues. Recently our Sensei talked with him letting him know he needs to pay attention to the waza and be more harmonious with the other members. I have practiced at other dojos and have never seen anyone except very beginners move the way he moves. Learning tenshin or tenkan movements I think are fundamental starting blocks for any Aikido practitioner. This guy has trouble with very basic movement. No one insults him during practice. They see he is not doing the movement right and give him tips or suggestions to which he completely disregards. This guys ego is quite big for some one with nothing really to show.
You are right this is a new dojo where he did not start Aikido. I think an important part of learning is adapting to different environment. If I go to another dojo, I understand there will be different ways of doing things and such. I will have to adapt and change or leave to find another dojo. It has been a year and there has been little progress and change in this guys attitude. How would you feel if you are practicing with this guy from another dojo who cant do the technique despite you not holding tight, give him a suggestion to do the technique to which he completely ignores you, then crank the crap out of your arm. Repeat every time you practice with this guy. I think this would frustrate anybody.