Do you have any pictures? Are the worms on his vent or in the poo on the ground, and how long had the poop been there when you saw them? What size are they? Sorry for all of the questions.
Vent gleet is a fungal infection in the digestive system, and it causes smelly runny poop, and the outside of the vent becomes very irritated and sore with red or white patches.
I would try to rule out his being pecked about the vent first, but it can be treated with antifungal creams, such as Nustock, clotrimazole from the dollar store, and oral Nystatin. You can do some butt soaks in warm Epsom salts or Dawn and water. Apple cider vinegar 1/2 to each gallon of water can acidify the digestive tract. Here is some info about it:
https://www.tillysnest.com/2012/12/vent-gleet-prevention-and-treatment-html/
https://www.backyardchickens.com/articles/vent-gleet-aka-nasty-chicken-butt.64647/
Vent gleet is a fungal infection in the digestive system, and it causes smelly runny poop, and the outside of the vent becomes very irritated and sore with red or white patches.
I would try to rule out his being pecked about the vent first, but it can be treated with antifungal creams, such as Nustock, clotrimazole from the dollar store, and oral Nystatin. You can do some butt soaks in warm Epsom salts or Dawn and water. Apple cider vinegar 1/2 to each gallon of water can acidify the digestive tract. Here is some info about it:
https://www.tillysnest.com/2012/12/vent-gleet-prevention-and-treatment-html/
https://www.backyardchickens.com/articles/vent-gleet-aka-nasty-chicken-butt.64647/