Ask and listen, then give feedback
What are your individual goals? What are the players' common goals?
Each gamer (player and GM) has goals. It sounds like it's time for everyone to reflect on what their goals are, and then discuss them to try to reach a mutually workable plan.
It sounds like you believe there is broad agreement that your GM is unfun. I'd likely start with informal conversations with other players about what they are and aren't looking for in the game to confirm your impression and clarify what folks want. I wouldn't be surprised if (even if there's broad agreement about what folks don't want) there may be variety in what they are looking for.
Next I'd talk to the GM privately. I might start by saying "I've had some thoughts about the game for me. Would you be open to discussing them? Is now a good time?"
I'd frame my feedback in terms like, "I've recently realized: what I'm looking for is X" or "...that I get frustrated quickly when Y." If you're really talking about yourself and your personal preferences, it may be easier for your friend to hear.
What are the GM's goals?
Before I've said more than a handful of sentences I'd pass the ball back to him again, first seeing if he has any thoughts or questions for you. Then I'd ask what his goals are. Here are a few GM goals I've encountered:
- Create opportunities for the player characters to have fun and be the stars of the story.
- Create a challenging mystery for my players to solve. Make it tough enough to beat that victory really means something.
- Craft and share a believable world with tough choices and lasting consequences.
- Enjoy tactical combat versus the players.
- Get to be in charge and make the decisions.
Is there reasonable common ground between them?
None of these things are necessarily bad, but the players don't like them.
That's a great insight. His goals may not be bad but given all the goals a GM might have, you need to make sure his and yours can happily coexist. I've seen great GMs and great players who really weren't a match for one another.
If so, would feedback be welcome and useful to help the GM better reach his own goals?
Let's assume his goals match yours well enough, but his actions haven't been achieving those goals for you. Fair enough; being a GM is hard. Here's a place for specific feedback. Depending on your relationship, it might be useful here to say something like, "Those goals totally work for me, but a few specific moments in the game have felt to me like they conflicted with those goals. Can I give you an example?" Assuming you get confirmation, give one specific example, e.g. "When X did Y, I felt powerless and frustrated." In some cases the GM may be eager to improve but not know how. Consider asking if he'd like ideas about some portion of his goals. Let him drive this conversation, if he genuinely wants to improve.
What if he's unimpressed?
Does he think the problem is you? Maybe encourage him to talk to other players too, and warn them that you're trying to reach agreement on some group goals.
Does he just not want feedback? Or want something different than you do? It might be time to thank him for putting so much time and energy into GMing, and to bow out of the game.
Good luck and let us know how it goes!