I thought it's going to be dangerous if he tries that move with someone using full strength. I thought it was just a simple mistake and he'd realize it soon so I didn't say anything the first time he did that. but it turns out that he's still doing that the when it's my turn again to pair with him (we were in a group of 5 people, and I have shown how it's supposed to be done when I'm tori). that's when I said to him, "maybe it's better to control and redirect the attack like this.." he then quickly dismisses my suggestion by saying (I'm quoting) "if you'd done that, the attacker would've smashed your face".. I was thinking, how would it be possible when I'm behind him and in control of his movements?
His concepts of how to move is different. Assuming he is somewhat good in his old art, I don't think you ever will convince him of the logics of aikido without taking the discussion to really deep levels. That won't be done in a class setting. Arguing over which way to do technique is better probably is pointless. BUT he should respond to "that's not how the teacher is doing it", and adapt accordingly.
You might think that if he sees that you (and several other people in the dojo) does it in another way than he does, he should realise his way is not what is expected. That might not be so, actually. He might have missed that point when the teacher showed the move - or the teacher's version might be slightly different (better) than what the rest of the people in the dojo are doing, so that the teacher's version look OK in his eyes but what the rest of you are doing not. And so he is trying to find a solution to how that technique should be performed.
So if you want to try to help him, I suggest the version "the teacher showed it like this", instead of "it might be better if you xyz". If he argues with that, then obviously the two of you will have to ask the teacher. If he insists in doing it his way although the teacher clearly showed how he wants it done... then the guy doesn't want to learn, is behaving poorly, and surely will drop out. Pointing out to him how he constantly (if that is the case) is doing things differently from what the teacher is teaching might help him along in his process of making that decision.