His poop is similar to diarrhea because its watery and his voice sounds different, and he does this like cough/sneeze sometimes. I feed him 16% protein grain along with cracked corn. I've had him for about 5 months.
Can you take him to a vet?
Photos of poop and rooster? If you have video upload that to youtube, then provide us a link.
I may have missed it, but how old is he?
He's not been wearing a No-Crow collar has he?
Look inside his beak for any obstruction, lesions or canker. Let us know how his crop feels.
If he is sneezing/coughing, then he probably has a respiratory illness, does he have any swelling of the face, watery eyes or mucous?
Tell us more about the protein grain, is that a poultry feed?
It's hard to know what's going on with him, if he has a respiratory disease, then you can try treating with antibiotics, just be aware that even if he recovers he will be a carrier of the illness - meaning he will pass it on to other birds.