Talk to him, preferably as a group

This is really the only solution besides "Bail on the game." They key here is to be respectful, polite, and try to handle this like friends.

"Confront" is not the approach you are looking for here. "Discuss" would be better. You want to approach this from the standpoint of mutual improvement of the game, not of "You are a bad DM and you should feel bad." The goal is to find out why this is happening, and see if you, the rest of the players, and the DM can all work together to make something you all enjoy. That is, after all, the goal of tabletop RPGs

You want to approach this in the context of "Hey, as we've been playing, it doesn't feel like my character really has the freedom to interact with the world and the story the way I'd like to. Is there anything I can do to help?" (Use your own wording, of course). You also don't want to ask for some massive change all at once; especially if your DM is new. Small changes, incremental improvements...that's what you want to shoot for

What you are experiencing is commonly known in the tabletop community as Railroading. (The DM has put you on the tracks, and you can only go where the tracks lead). There are several possible root causes of this.

DM may not be good at Improvising

DMing is hard. Don't let anyone tell you that it isn't. The goal is to build a world full of interesting characters, make them all react appropriately to the characters, allowing your players freedom to interact with your world however they wish...and still weave an interesting, coherent plot for them to enjoy. And many times, you have to make all of this happen on the spot--pure improvisation (otherwise, you have to call a break to consult your notes and figure out how to deal with whatever the characters just did).

It's like directing a movie where nobody gave the main characters the script.

Railroading makes life a lot easier on a DM. You have built your plot, and any deviation from it is swiftly nudged back onto the rails. This lets the DM build out their plot and plans like a screenplay or novel.

This is very common in newer DMs, because they get excited and plan out their story, and then don't know how to react when someone does something they didn't expect. So they shove you back into their plot and carry on, or prevent you from jumping the rails in the first place.

This is an acceptable problem for a DM to have, because they can improve. If this is the case, then you need to talk to your DM about how you would like a little more character agency. Make sure you don't have your characters jump too far off the rails (cooperate with your DM, so you aren't making him miserable). This is a growth opportunity for your DM, and it might be uncomfortable for him. If he agrees to try to loosen the rails a bit, be patient with him...he's learning to improvise better, and may need some time to think.

In the specific case of 'boss fights' you were talking about....keeping bosses alive is a real challenge for DMs sometimes. It sucks when your Big Bad Evil Guy goes down to a flurry of lucky crits...and a DM who isn't good at improvising is going to have a hard time restructuring their plot--or slapping together a way for their villain to survive/escape without just blatantly lying about what happened.

DM has a very different playstyle than you.

Some people like cinematic, arcadey tabletop games. You play through them like they are a video game, and enjoy the cool cinematics that your DM describes and plays out for you. Your DM may be one of these people and simply not have thought of the fact that you all may not agree on style.

This is an easy discussion to have, because his tastes will quickly become apparent, and you can ask for tweaks and changes to make it a bit less arcadey.

The Same Page Tool can be useful in this case, to figure out what everyone likes

DM has the wrong focus

This is going to sound a little harsh, because I can't figure out a better way to phrase it...

Another cause of railroading is that the DM may have it in their head that they are telling their story, and you players are just along for the ride. Instead, the focus of tabletop RPGs should be the DM and players working together to create the story.

A very common symptom of this can be if the DM has NPCs who are hogging the spotlight. You end up feeling like you aren't the main characters in the story...because the NPCs are the entire focus of the story.

Sometimes this happens with new DMs who, again...just don't have the experience to know better. They don't know how to tell a story where they can't control the main characters. This is another one that can be solved with a similar discussion to the possibility above.

On the other hand, sometimes you do get DMs who are not interested in making the story be about your characters. This is, thankfully, fairly rare...but it does exist. This is one of the 'bad kinds' of DM, and many are intractable. In this case...you really don't have a lot of options. It doesn't sound like this is your DM--but it is a possibility.

DM may not know any different

If your DM is new to tabletop games and DMing, his experience with gameplay storylines may be drawn from video games. That structure of 'cinematic followed by action' and 'invincible boss until boss fight' could just be the gameplay structure he's used to. The idea of how much agency players have in tabletop games takes some adjustment for people new to the format.

Wrap-up

This is the most common causes, in my experience, of railroading DMs. Hopefully it can help you in your discussions with him. Just remember...talk about it like friends. You're trying to have a good game together, so try to solve your problems as a group. If he is a novice DM, your goal is to work together to help him improve--thus improving the game for all of you.

"We can't talk to this player. So I don't know what to do."

I think you just took away most people's #1 answer.

If you can't talk to someone about issues, and you can't remove them from the game, then your choices are reduced to 1) Stay, or 2) Leave.

Stay. If the overall fun outweighs the annoying bits, then stay. Ignore the behavior as much as possible, try not to let it get to you.

Leave. There's an old saying: "No Gaming is better than Bad Gaming." Fun is only fun as long as it's fun, and then it's not fun. If it's not fun and you can't make it fun, then it's more like work, except no-one's paying you to do it. So why would you?

There are also some middle ground options, such as:

• Opt out when it's their turn to GM. Find something else to do.
• When you GM, don't use the things you know will bother them, on them. That might mean pulling spotlight from them, but so be it. This is a natural consequence of them being difficult, that you find ways to make it easier.
• When you're co-players, don't engage with their character. Disengage quickly when they engage you. This can be difficult do do smoothly, and personally if someone is so bad you don't want to engage them, that's just another mark in the "leave" column. But it is possible.

That being said, I'd make darn sure I've exhausted all the talking options first.