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Just make sure he's eating and drinking. If he is, he'll be fine, just small, so keep him with the others. The think about quail is, if they're not eating and drinking, they won't live more than a couple of days, so if he keeps going, he should be fine.
I've got a runt from my last batch too. He wasn't noticeably smaller when he hatched, but he's less than half the size of the others. He's eating and drinking fine, so I think he's just a runt.