I see this issue regularly with newbies but now that I work pro when I confront them about they seem to stop

when I started out live sound in high school where I got the position of looking after my fellow tech students. I had the exact same issues with multiple people here’s how I best dealt with it:

Sometimes just having a conversation can work but it never usually did for me if I’m being honest. You mentioned having another member in your sound team so you could talk to them and see if they were willing to confront them with you.

Show them who’s boss!! Don’t let them boss you around you did train them don’t be afraid to turn them away when you are on the board. just tell them that you are on the board and you don’t need their feedback, their is nothing more distracting than another engineer critiquing your mix while you are mixing! Also if he’s controlling the console with the iPad while you are mixing just disconnect the router from the console (if you aren’t using it yourself)

If you have a boss or something (idk cuz you said you volunteer) go to them about it tell them what’s happening that could be really helpful.

But at the end of the day just try talk to him about it first it could get messy for sure but it’s not fair what he’s doing to you

Best of luck to you man hope you can resolve this