Need help....

Hey guys I have a two year old rooster who has suddenly stopped crowing and has become inactive. Also his comb has turned black a bit. This has started all of a sudden since yesterday. He was fine before and the rest of my flock is doing fine too, its only him most probably coz he used to wander off alone in the bushes during the day alone. As per my friends suggestion I gave him a sulphamethoxzale drug after which he did crow once and ate some bits but his crop is still puffy. His poop is greenish yellow in color too.

I need your help guys hes the best I have. Everyone in the family love him and I know hes a fighter but I need guidance on how to deal with it and what it could be.