I will go through this with my super special awesome red pen of criticism.
Please note that any of my opinions are just that - opinions.
Jon looked down at the broken pieces of glass strewn about him. He also looked down at his leg
I'm not sure I like the "He also" part. Seems a bit awkwardly worded. Consider making it a compound sentence.
He also looked down at his leg, which had splintered in an ugly way upon impact with the ground. There was no bone showing, so the fracture was simple enough, but it was there, so it was bad enough.
I wouldn't say it was ugly. Show how it was ugly. Bad enough for what? To be bad? Say how it was notable bad.
he brackish weather didn't help his current state of mind - he was being made out to be a ribeye, a cut of meat on the grill, unable to move, and unable to call for help.
I didn't know weather could be brackish. The metaphor isn't bad in itself, but I don't see that it describes his state of mind so much as his situation. Consider rewording.
Jon's breath was stolen.
This bit doesn't seem to have much to do with what follows. I would try to find a more natural place to put this.
Jon kept trying to move inside, to get to the phone, but while his right leg was fine, his left was swallowed in an inferno
If you mean a literal inferno, consider telling the reader about that earlier and with a little more emphasis. Otherwise, I wouldn't use so many metaphors starting a novel in medias res. It' may confuse readers (QED).
he couldn't do it. He couldn't do it. He couldn't do it.
You're messing with dangerous forces here. Such repetition can be poetic, but often cheesy. Also note that in a larger work, you can use repetition on a larger scale, i.e. a phrase or idea repeated throughout the work. Like in the Odyssey. I'd consider altering this.
He was being buried in his incapacitation and his inability to yell, to press the panic button.
The exact meaning of these metaphors is more apparent. Consider keeping them.
It wasn't until he could start breathing again that events began rolling in a favorable way.
That get's the meaning across, but it isn't pretty. Reword.
The neighbors had stopped playing water Knockout long enough to hear Jon's cries of pain.
Lose the had. A simple past tense shows the action better, I think. Capitalize water (if the game is called Water Knockout).
A wispy adolescent climbed the fence and saw Jon dragging himself through a patch of bloodied grass, and called for the phone so he could alert the police.
Consider making the last clause its own sentence (or an independent clause with a semi-colon. Same thing.) That, or drop the bolded "and" in favor of a comma.
He fell into an inset garden, and a tomato cage punctured his leg.
This is a difficult sentence. I don't like your wording, but I'm having trouble finding a better way. I'm no good at showing simultaneous action. Maybe consider dropping the "and" (followed by a bit of rewording).
After wrestling it out of his calf, Jon could start moving.
You tend to say what Jon could do more than what he does (passim). Consider showing more action.
Also, would you
wrestle a tomato cage out of your calf? Doesn't sound fun, or like the best way to handle the situation. It does, however, show the desperation of the events. Still, consider changing.
He was thankful that it didn't puncture his femoral artery, but as far as flesh wounds go, he was in a bad situation.
I think you have adequately shown that his situation is bad. No need to restate it. This whole sentence strikes me as unnecessary and a bit out of place. Consider a different way of saying that he sustained no mortal injury, or allow the reader to infer.
Also, with respect to the femoral artery thing: Dangerous forces, once again. Trying to display medical knowledge is a great way to look like a jerk if done wrong, and this sentence seems to go out of its way to reference an artery.
He had to crawl up some and then past his porch, over to another inset area, and then out of the fence gate.
As I mentioned before, don't say what he had to do, say what he did.
But as every blade of grass was entering and torturing his wound, all he would be leaving was a trail of blood.
Don't start sentences with
but. Seriously, don't.
I don't really see the relation between the two clauses. How does blades of grass entering a wound cause a trail of blood to be all he leaves?
Also, you might want to show more action instead of hypothetical action (passim).
He had barely made it to his back porch when it happened again.
I'd like a colon here, but I overuse colons.
His skin glazed over, shielding him from the temperature, and he couldn't find his arms anymore.
I'd make these two separate sentences, i.e. "His skin glazed over, shielding him from the temperature. He couldn't find his arms anymore."
he mown grass of his backyard faded from his nose, his shirt fell away from him.
I think you mean mowed.
The metallic taste of blood disappeared, and he barely had time to dance in the ambulance's disco lights before they, along with the siren's whine, were swallowed by a puff of smoke.
Make this two sentences.
The path in front of him was long and made of fragments of light.
Ooh, that's not good. Why not "The long path in front of him..." This way sounds awkward and is needlessly wordy.
Each step brought about an increased level of heaviness,
I'd suggest rewording this. Sounds too mechanical.
But he had to keep walking.
Don't start sentences with
but.
Eventually, all he could see were the stars he was walking on, and soon after that, he was falling through the floor, towards the bottom of the ocean.
I'd make this two sentences (eliminating the bolded and). In fairness, I'm in a two sentence kind of mood.
Was that along the lines of what Uncle Alt wanted?
