It could be a number of things, but with his darkened comb color, it is obvious that he is not getting enough oxygen. We are not vets here, but he could be suffering from a respiratory disease, worms that are partially blocking his airway, or a fungal disease (aspergillosis) that has affected his respiratory system. I would try to see a vet familiar with chickens right away. If not, I would give him some wormer, SafeGuard Liquid Goat Wormer (shake well) 2 ml slowly over a minute or two, for the next 5 days. That treats gapeworm and capillary worms two of the worse worms, plus several others.
You could try some Tylan 50 or 200 if your feed store has any, and give it orally. Use a syringe and needle to get it out of the bottle, remove the needle, and give it by mouth. That would treat mycoplasma (MG.) Look for dust and mold in his coop, and remove any. So sorry about your rooster.
It is upsetting to not be able to help him. VetRx is only herb oils plus camphor, and will not cure anything. Make sure that he is getting enough water, and you can take a small portion of feed and make it ver wet with water to more more into him.