Chicken Little

We've moved our chicks to a larger box and have had to replace their 'baby' feeder with a much larger feeder. The chicks are growing at lightning speeds. However, one chick isn't growing. He's almost the same size as when he arrived almost two weeks ago. We've affectionately named him Chicken Little. We thought perhaps the other chicks aren't letting him eat, so Dan put a smaller box inside their big box and put Chicken Little in it with his own food and water. He seems to breath very heavy and is almost dazed. He's also had a pretty bad bout of pasty butt. I check on him a few times a day and clean his behind and dip his beak in his water and food so he knows it's there. I've also been giving him yogurt in case he hasn't developed enough good bacteria in his system. Hope he'll pull through so we don't have to cull him. :(