If the water parameters are good, which seems to be the case, and there is not a lack of oxygen (or your other fish would also be gasping) It's possible it's the shock of being transferred quickly from his bag water to the tank water.
I don't think there's much you can do now except keep him very quiet, switch off the lights, and give him a chance to recover. I hope he will.
Next time, while you float the bag, try letting a very small amount of tank water into the bag, a sip at a time, at 10 minute intervals, and removing a bit of bag water too, very small amounts, making sure none of the bag water goes into the tank. After enough time has passed, and you have completely and very gradually replaced the water in the bag with tank water, release him gently.
Matching temperature is important, but it is not always enough.
I don't know if this is what is wrong with him, but it seems the most likely reason.
Good luck, I hope he gets better
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