The "His lungs were closed!" bit is especially idiotic. That's not how asthma works. If the inflammation had become severe enough to actually knock the kid out, you probably would have seen some terrified faces, some discoloration, and some serious flailing just before. The fainting would have occurred due to lack of oxygen to the brain. It should have taken longer than it did for things to get to that point, sure, but I can understand Shyamalan taking advantage of a little plot convenience.
After the kid lost consciousness--and, again, it should have taken longer to get to that point--it's likely the bronchioles would have relaxed a bit, which would have resulted in some shallow breathing. Whatever the case, his lungs shouldn't have "closed" entirely. If they had, and if the kid truly would have entered into respiratory failure, I'm not sure an adrenaline shot would have been the way to go. (Doesn't he give the kid a shot? I haven't seen the movie since I saw it in the theater, so I could be misremembering.) It can restore blood flow to the brain a bit, and it can relax the lungs to some degree, but it's jarring as hell, and most medical professionals would probably recommend that other things be done first.
I doubt the kid would have regained consciousness gently as a result of the shot. He probably would have woken up in pain and with the shakes, and he probably still would have been fighting for breath. That almost certainly wouldn't have been the end of it. He would have needed a puff from a rescue inhaler or some oxygen or a nebulizer.
Asthma faints are terrifying and painful. I've been there.
The Gibson character takes his time about getting the kid away, which isn't all that smart. If we choose to believe that his lungs are actually "closed," it should be a given that new oxygen's not getting to the brain, which means brain death is about to start setting in. Why does the Gibson character take him all the way outside? Why not just leave the room? Why not go wherever all the kid's asthma meds are?
Also, the alien straight up covers that kid's face in gas. It probably should have left some residue--especially in the nostrils. It's alien gas, of course, so it's not like we can know its properties. Maybe the Gibson character should have taken a washcloth to the kid's face or held the kid under water or something. Why not? What's the kid gonna do? Drown? Impossible! HIS LUNGS WERE CLOSED!
The more I've thought about it over the years, the more I've wondered if maybe Shyamalan is also making fun of Christianity. It surprises me that more Christians aren't insulted by it. It takes an incredibly long list of signs (SIGNS! HEY, EVERBUDDY! THET'S THE NAME UV THE MOOBIE!) for the priest to regain his faith. It takes him believing that his god did a metric shit ton of orchestrating. His god had to orchestrate his wife's death so that his wife would say some stagey deathbed (or, rather, deathtree) bullshit to remind him to tell his younger brother to beat an alien with a bat years later. His god also had to work in the daughter's weird-ass fetish with the filled glasses of water. The Gibson character also makes it clear that he comes away from the experience believing his god gave his son asthma just so the son would survive the alien spray.
When I say Gibson's character makes it clear, I mean he actually *says* it. He says something along the lines of "That's why he had asthma!" Seriously, man. How can Shyamalan not be poking fun? He makes it so that Gibson's character appears to believe his god SENT AN ALIEN across space to bring him back to his faith. Gibson's character must believe that his god really, really worked overtime to restore his faith. He must feel he's pretty important for his god to do that for him. His god didn't do that for everybody, after all. He's special. And he has a son with god-given asthma to prove it. (I hope Gibson's character's god relieves the kid of his asthma after this. He no longer needs it. It served its purpose.)
It should be noted that Shyamalan has the temerity to show up in his own movie to tell the Gibson character that he thinks the aliens are afraid of water. That's right. Shyamalan SHOWS GIBSON'S CHARACTER THE WAY. This only strengthens my belief that Shyamalan's story choices were motivated by arrogance. (He would revisit this in a roundabout way in Lady in the Water. Once again, he shows up in his own movie--this time as a character whose writing is so powerful it can create. He also includes a straw man movie critic [played by Bob Balaban] who is shown to be bad and wrong about everything.)