Unfortunately, I have no experience with chick diseases, so my expertise ends at asking for pictures.
If you can, I'd isolate the chick rather than the rooster. He's sick, and continued exposure might cause problems with the other chicks.
Is your weather warm enough for you to tether your...
Can you post some pictures of chick and poop, and include a description of any other symptoms?
Watery poop is not a good thing. Have you tried electrolytes and vitamins?
I'll tag the experts
@casportpony @Wyorp Rock @WVduckchick @Eggcessive