The development department I belong to works closely with a certain expert from the testing department. This particular colleague possesses a unique set of skills for our company (but not in general), so you could think of him as our in-house specialist for certain topics.
The problem is that this expertise has greatly inflated his ego to the point that he cannot accept that he is wrong about some quite important subjects. For example, he often compares our products with the ones of the competitor and criticizes us, even though it seems that he misinterprets the competitor information. This leads to increased effort and wasted time in the development to satisfy a standard that not even our competitors can reach.
A certain phrase in the competitor's manual has sparked a lot of unnecessary tension between him and our department managers. It has lead deep into a technical discussion, where he feels safe to claim whatever he wants. After getting acquainted with the technical bits and pieces, I have reached the conclusion that the disagreement is due to a misinterpretation of the text, stemming from the fact that none of us is a native speaker of English. The same goes for the competitor, however the text of the manual seems to be otherwise perfect besides this... particularly convenient mistake. Unfortunately, this phrase is not yet seen as "bad English" from the rest of the group, so this argument has not really been discussed. I have gathered several sources that partially or completely support my claims, but he has been known from another situation for rejecting evidence shown to him.
Recently, I briefly expressed my disagreement in front of several other colleagues and my manager decided to move on with other topics of the appointment, since this topic has been discussed many times before (not in my presence). I apologized and moved on, but my manager was clearly ok with me intervening, since this is a pain point for us. In the end, I approached the expert and offered to have a meeting to see his point of view and it seems that it might just happen.
Neither I nor he have any management responsibilities. I know that many people will argue that this is a problem of my manager and I should not get involved or that it would be difficult to change the mind of such a person, but I would love to try my best to get to a solution due to its high impact while demonstrating to my manager that I am working on my communication/management skills.
My questions: how can I approach a colleague with unreasonably strong opinions about a subject open to interpretation, with a goal to plant a seed for a healthy amount of doubt? what are some good strategies for getting him from 100% sure to 90% sure about this topic? how can I get him to pay attention to the evidence? how can I gain his trust and prove that I will help him save face in case he changes his mind?