How old is he (I'm sure he is a cockerel?) Cocididosis could possibly be a problem. Symptoms are lethargy, diarrhea (sometimes it may have blood,) poor appetite, weakness, sitting or standing puffed up, and ruffled feather. Treatment is amprollium (Coxoid, Corid, Amprol, Ampromed) in the water for 5-7 days. Are you in the UK--in the UK and in Canada it may have to come from a vet, but in the US, most farm stores sell it in the cattle medicines.
How are his legs if you stand him up--is there any lameness or paralysis of one or both legs?
He is 6 months old. He seems to be doing some better. Drinking and eating. Still fluffed up. We are in Canada. I have purchased some diatomecous earth. Mixed in with his food just in case he has worms.
DE won't help with worms. It helps to dust it around the coop to keep away mites, etc. and some claim it can work on internal parasites but if you suspect worms, you need to use a dewormer. Make sure he keeps eating and drinking... you might want to research cocci more too since it's fairly common and treatable.