Please, that just brings up to many painful memories of 6 years wasted trying to find answers to questions that have no answers. (So...There's a cork in the ground? That's what you guys are going with! Douchebags!)
Inception leaves the audience wondering if they were still in a dream or not, but dreaming was a huge part of the story. In my story, storytelling is a huge part of the story, thus if I conclude with the audience wondering if the entire story was just another story inside of a story, it works. But either way, this shouldn't be your climax. It's more or less 'food for thought' for your reader to think about and discuss with friends after they are done reading it. "So was he still in a coma on not?" "Did he wake up from the coma in time or did he die and this is some alternate universe he's stuck in?"
If I mention the entire season of Dallas when Patrick Duffy decided to leave, would I be showing too much of my age? As for the actual question, I always feel ripped off when I read something like that. I like the other ideas about somehow finding out that he's in a coma and having to fight out of it. The other thing you can do, is have him come out, and then go on about his life. . . but in the closing paragraph, reference something that he sees, or that happens, which shows the reader that it wasn't really a dream. In other words, while his body was still in the bed, somehow, he was out doing everything that your reader has spent the last 300 or so pages reading about.
As people have mentioned, one of the largest issues with this type of story is the reader feels like no progress is made anyways. Let the car crash be a very minor part of the story. Don't even dedicate a full chapter to it. At the end, after he succeeds and he's made these new friends and the antagonists have been vanquished. Give him something. A feather, a necklace, who knows what. Make it an important part of your story at some point. When he wakes up. Everyone in his family is all happy to see him awake etc. He opens his hand and whatever thing that was given to him is still there. It gives the reader a good feeling, like maybe it wasn't all a dream after all and gives them a sense some progress was actually made.
