Comb turning black, tail feathers hanging

Hi All,
My family and I are new to raising chickens so I'm looking for any help I can get! My son hatched a chick for a 3rd grade class project this May and managed to con my husband and I into keeping it and getting the chick some "friends." So, we ended up with 1 rooster (from the class project) and 3 hens (from a farm supply store). We have no idea what breed the rooster is but the hens are Barred Rock, Americauna and Rhode Island. We built them a movable tractor coop that has a run underneath. The coop itself is about 4x4x3. The run is about 12x3x2. Everything had been going just fine until this last week.
Usually the rooster would start crowing around 6:00 every morning without fail (even before we had let them out into the run). He'd always be the first one out of the coop in the morning. His comb was a pretty red, his tail feathers were all held high and he strutted around the place like he owned it.
Within the last week his comb has started turning black, his tail feathers are hanging, he either sits in a corner or stands in one place. He looks depressed. Usually when I would reach into the run he would jump at me but now he doesn't even really move. The hens are running right over him. He's the last one out of the coop. This morning I didn't think he was going to come out at all! We live in NE so the temps have gotten into the low 30's lately. I read that the blackening could be frostbite, but I don't see how that could be when he's in an insulated (and ventilated) coop. And if he's already getting frostbite when temps are in the 30's, we're in trouble when winter really hits and it's well below 0 for several days. He seems to be drinking just fine. Oddly he won't eat the feed (layer crumble) he had been eating but will eat other things like tomatoes, grapes, watermelon, warm oatmeal with bananas, etc. just fine. He does have a little diarrhea but his droppings in the coop look normal.
All the hens look just fine. They're all laying eggs like clockwork. They're combs are fine.
I have no idea what to do. Someone suggested that I try a shot of penicillin from the local Tractor Supply Story but I'm not comfortable doing that when I don't know much about chickens. Any suggestions?!

Thanks much!

Darkened combs like his are often a sign of poor circulation. He may have some sort of heart problem, in which case there is probably little you can do. I would keep making sure he is eating and drinking and avoid stressing him out too much. You might also try separating him from the hens so he isn't getting trampled or picked on. A little time away from all of that stress may allow him to regain some strength.