I'm currently leading an amateur team of around 20 working on a relatively not-so-small side project, and we plan to deliver the result to thousands of users after a month or two.
However, there is one member (who theoretically should be in charge of UI/UX) that I am worried about.
In my own perspective, I think the member is taking way too much ownership; the member seems to take the whole project as his own baby, take part in almost all sub-teams, use his own understandings to decide what to deliver, and often turn down others' suggestions.
I did not notice this in the early stages of the project; I thought the member was just very supportive and willing to contribute. However, during the past few weeks, this situation has been more and more noticeable; he now also commits as a frontend and even resigned from his own job last month to commit even more to the project. He expressed quite a few times that he wants to deliver the whole project on time and ensure that the output matches his expectations.
While I sincerely appreciate the member's contribution, his excessive ownership and insistence are causing some trouble. He tries his best to make sure the UI team delivers what he expects, not what I or other members think our users would expect. We do have Product Requirements Documents (PRDs), but they are now pretty much just a side reference to him, ignoring detailed descriptions that he does not agree on. He also vetos some PRDs if they violate his own expectations or understandings of the project, and is reluctant to adapt to other points of view.
Unfortunately, we are not in the kind of (atmosphere? society?) that encourages people to directly express their concerns; most members are undergraduates, so they don't naturally feel confident to directly speak out against the senior members. (Yep, some Asian culture.)
I am not sure what to do; my past career (around 3yrs) is mostly as a developer, only with a short time working as a substitute PM while working for a startup. I just really don't want my other members to find out that our users don't like the final results after working hard for months...
Edit: a bit of background
The project was a students-maintained service in my alma mater, mainly to serve students and professors for smoother lecturing. Since demands/users have grown from dozens to thousands, the service is both functionally and technically outdated, and really needs some refurbishing. I talked to professors who are in charge of the system, and I volunteered to organize a team to make a brand new version of the service.
I initiated this project, and it is totally volunteer-based. At very early stages I decided all things, and after the member joined we discuss most requirements together. The member volunteered to conduct some user interviews (and I willingly let him), and we will discuss the requirements together.
I guess I have too many roles (I'm also the backend tech leader), so I really do not have enough time to work out all the details, and in the middle stages I usually let the member decide on some details by himself, given that we already reached consensus on meta stuff.
In the past few weeks, the member started to refuse to accept my ideas and we almost had a pretty serious quarrel privately. I thought I was being too (verbally) aggressive and tried to give in a bit, but things seem just to get worse. Now the member strongly insists on his designs and turns down most suggestions from other members. I am still the official person to make decisions, but I'm losing control of the UI designs since he will simply refuse to deliver what I expect and do what he thinks he should do. This recently resulted in inconsistencies between design mock-ups vs. PRDs + backend implementations, and frontenders are sandwiched in-between...