Help! my chicken keeps sitting down

When i first got my chickens, they were 7-8 weeks old. They are 14 months Old now.

Anyways, since i got them, I noticed that the bbs orpington wasn't scratching and was sitting a lot. He walks a bit and then sit down. He eats, he drinks, he dust bathe, he sometimes roost but very seldom.

At first i didnt mind because he was walking fine but today, i noticed that he is walking awkwardly. As if he's limping. He still drinks, eat and dust bathe. I picked him up to check on his legs but i don't see anything wrong. I checked on his vent and saw wet poop stuck on some of his vent feathers. I remember that one of the chickens used to bully this poor little guy but the bully chicken is already dead.

What's wrong with my chicken? Any help is appreciated.