Co-worker disagrees with my ideas in private discussions but presents them in front of the boss

Hey, I'm new to the workplace (18F) and need advice dealing with a particular co-worker (19M? don't know age but a little older than me). I won't say what I do for work, but it's a white-collar office job done remotely due to COVID over the summer, about to end in a couple of weeks.

In absence of the boss, when I propose an idea (e.g. add X to the project for Y reason), there's this one co-worker--we'll call him Jim--that verbally disagrees, and some of the other co-workers in the group will agree with Jim. We'll have a back-and-forth, entire group agrees not to immediately add X to the project and that we'll think about it after we finish some other things, and we carry on with our work.

Then when the boss drops in, Jim presents my idea as his own ("We'll be adding X to the project for Y reason"). I ask Jim and some of the co-workers that originally agreed with Jim what's up, and they say that they changed their mind and they will add X to the project??????

Sometimes, when the boss drops in and I speak before Jim ("We were thinking of adding X to the project for Y reason"), Jim says something along the lines of "Actually we weren't because <reason>, there might've been some miscommunication on our part so sorry autumntealeaves." Boss tells our group to settle on an agreement before getting him involved, and leaves.

I could just save my better ideas for presenting in front of the boss and limit private discussions to more generic comments. But when I do that, my co-workers complain that I didn't bring up the idea beforehand, and my boss tells us to discuss the idea among ourselves before getting him involved, and leaves. Co-workers write evaluations for each other every so often.......

Also, sometimes, what I consider a generic comment on a small detail Jim will still present it in front of the boss like it's golden. And as always, before the boss comes around, in private, Jim will (diplomatically) argue with me for a long time about this small detail as the rest of the group watches. By small detail, I mean really really small aspects of the project like formatting and style.

What makes this more frustrating is that Jim has a very diplomatic/charismatic speaking style, even more so with the boss. In my boss's eye, he looks like a great team player. I obviously need to confront, but do it in a way that remains classy.

Advice?

TL;DR: co-worker disagrees with ideas in private discussions but presents them in front of the boss, the ways I've attempted to deal with this result in boss telling my group of co-workers to discuss it over before involving him, boss is very hands-off about our approach to the project

Submitted July 30, 2021 at 02:26PM by autumntealeaves https://ift.tt/3C4XuQu