When my roosters used to eat a layer they were often dead by age 4. It always started with lethargy and looking old. I would switch your rooster off of layer ASAP and see if it helps. I recommend a non medicated starter grower or an All Flock ration. My oldest buff Orpington rooster was 4 when I switched and I thought he wouldn't live long but he turned around and is still going at 7. Layer has too much calcium for roosters.A mixture of layers pellets and corn
Otherwise others have given you great advice.