His comb does have a purplish tinge and he seems a bit dehydrated to me, which could be due to the diarrhea/watery poop you noticed.
He might need a thorough deworming and some rooster booster afterwards.
What exactly are the ingredients of "game cock" feed? Does he have access to grit to help...
Hello and welcome to BYC! :frow
How old is he?
Did you check him and the coop for poultry mites and/or lice? It's the time of the year ...
What are you feeding?
How does his poop look? (Pictures?)
Feel his crop: is it bloated and squishy or rather hard like a ball?
Does his crop empty...