How to care for sick chick?

I have 4 silkie chicks, they were all born within a few hours of each other. When i got them at a day old, they were all small and didnt move much. Now its been a week, and three of them are active and growing big but one is still the same size and kinda just sits around. I put it in the food bowl and he eats and ive seen him drink but he doesnt move much. He's not being picked on but isnt growing. Should i seperate him and maybe make his food a little damp to encourage him to eat? I really dont want to loose this chick (they are just pets) and will take special care of it if it needs.