One of the chicks is sitting down, tipping its head up and gasping. What’s wrong?
Are his eyes closed or open? When he gasps, is there anything in the beak, like fluid? And is he sitting back on his knees, chirping at all, eating or drinking? How weak is he?
Last year, I had a Sebright chick who acted like that. If he was flipped onto his back, he couldn't right himself and just kicked frantically... he survived for about two days. We didn't have a brooder lamp, so we improvised with a contraption involving an insulated cooler, a heating pad, several towels, and a tissue box. Anyway, we realized that my small desk lamp had an incandescent light bulb in it and tried to keep him warm with that... he perked up a little, but unfortunately didn't make it.
I don't know what was wrong with him, and I don't know what's wrong with your chick, but the advice that I have is:
Check him for any injuries or other issues, like pasty butt, and treat those.
Keep him nice and warm (obviously). I'd separate him from the others so that he won't get stepped on. What we did with Turtle, the Sebright chick, was we put him in a small container (like a sprinkles or cream cheese one) and put that on top of the lampshade. If you do this, make sure that the lamp has an incandescent bulb in it so that it gives off heat. If you don't have an incandescent bulb, then put him in the container under the brooder lamp, maybe raised a little but (like putting the container on top of a brick or a larger upside-down container, to get him closer to the heat). You might want to line the container with a paper towel, as well. However, if he doesn't seem cold, then just put him in the container and scoot it a bit towards the lamp, so that he isn't too cold without the others, and go ahead with the advice below.
If he isn't eating crumble, then try to give him a little molasses or raw egg yolk by smearing a drop on the side of his beak with a toothpick or some such thing.
If he isn't drinking, then see if you can get an eyedropper into his beak to put a bit of water in, and see if he swallows. Try to keep his beak clean, and if there's a big drop fluid inside his beak that might be blocking his airflow and he isn't swallowing, then try dabbing it with the very corner of a tissue to absorb it.
Monitor the little guy closely, making sure that he isn't too hot or cold and moving him towards or away from the lamp accordingly.
Sorry that I don't have any more specific advice. I'm not an expert on chick care, really, and am just relaying what we did with Turtle. Not sure what's wrong with the little guy, and I hope that he pulls through.
Please keep us updated, and good luck to you.