Thank you for reaching out to Your Office Mom. This sounds like a difficult situation in your workplace. Try not to let this overzealous co-worker upset you. It sounds like he is trying to take control, either because he is a control freak, wants to move up in the company, or he is a bully.
Privately document incidents so you have a record of his behavior if it's needed. At some point, he should give up, or go directly to the GM and owner with his wonderful ideas. My work advice includes three different options for you to consider:
Try taking a direct approach, and push back. Next time the guy gets bossy and wants to define or change your roles, or tell you how to do your job, tell him he needs to talk directly with the GM and owner or bring it up in the next team meeting. Then, leave it at that. Ignore him, and, keep pushing back with this response consistently.
You might also give your co-worker feedback directly, but if you don’t feel comfortable, or think it will be well received, probably not. Just a thought!
Talk to the GM or owner. Explain the young man's behavior is persistent and you're primarily concerned about how it may be affecting the business. Tell him you understand the open door policy, but since this is impacting clients, you chose to step up and talk directly to them. Explain that you are frequently handling irate client calls and need to know how those should be handled (have specific examples to share).
If there's push back ask when you can discuss the topic in a team meeting because the longer it goes on, the more you become concerned about interpersonal dynamics on the job. Your goal here is to get the boss to take ownership, realize it's a big deal, and recognize they need to intervene since the team approach didn't work out the first time. If you make it more about the business impact and not so much about the team member, you may get better results! Money talks.
Or, you can get the topic on the agenda for an upcoming team meeting and bring it up. That may sound a bit scary, but if you have to resolve conflict as a team, it may be your only option. As long as you and others are prepared with specifics, bullet points, etc., and know what you expect as an outcome, you can discuss it and build a case for change.
I hope one of these options works for you! Check out the advice column topics for other insights. Good Luck and keep me posted.