Welcome to BYC. Sorry that your rooster is sick. How old is he? Do you have another rooster in the flock? Pictures of how he lies or sits can be helpful. Do you know if he could have been injured? I would feel of his crop to see if there is food and water in there, and that it is not full and hard or puffy. Look him over under his vent and elsewhere on his skin for lice or mites. Has he lost weight recently. Are his eyes clear and normal looking? How do his poops look?
This time of year we tend to hear about Mareks disease affecting chickens in their first year. Mareks can make them lose balance and have lameness in a leg or have a drooping wing. Other diseases may look and act like Mareks, so it is best to keep him near his food and water, and offer it up to his beak throughout the day to keep him nourished. Chicken feed mixed with water offered would be best for him, but you may offer him chopped egg or things like, mealworms, chopped liver, or tuna in small amounts. A chicken sling may be good to get him upright and keep him cleaner while having food and water in front of him within reach.
Here is some reading about Mareks disease:
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/