Well first have you examined him well? Check his feet and toes and then his legs to see if the limping problem is evident. Being thin could be from being in pain and not feeling like eating or it could be from worms. We need more info.
He was born with a deformed foot but that's never handicapped him before. he's our dominant rooster :/ he's also never limped before he's about 9 months old. And already crowing. He eats constantly, because we feed him constantly. We are currently keeping him alone in the warmth of our house nice and cozy
Try worming him and checking the leg for signs of infection, laceration or infestation. You may also want to examine his crop. First thing in the morning before he gets anyfood, feel his crop for any food in it. If the crop hasn't emptied then you may have a problem. Your poor rooster may have impacted crop (please research it because I don't feel like explaining it). This would explain the heaps of eating but skinny body. The limp may come from the crop sitting uncomfortably or it may be a completely unrelated problem. Also research bumble foot and if need be please ask for advice. I know how painful it is to be helpless when your chicken is in distress.
1. Why would you use a cock with a birth defect?
2. Are you not afraid you could be genetically adding that weakness in your lines?
3. Post a picture of the injured foot
4. Feed a few ground up pumpkin seeds ..will not hurt and only help
5. Add 1 tsp ACV/m to drinking water...
'ACV' does good things, it reduces harmful levels of bacteria, stripping the mucus from their systems, further improving nutrient and vitamin uptake, it boosts the immune system, and can't hurt your birds, in any way. Vinegar - is perhaps one of the natural ingredients that many might call an all-round treatment for many conditions. Vinegar contains vitamins such as beta carotene, minerals, amino acids and the aforementioned enzymes containing ninety different components. Vinegars effectiveness includes: treating gas in puppies, kittens, and chicks; preventing urinary tract in male cats, formation of mineral stones and helps to dissolve hairballs. They claim vinegar can lengthening life, improving hearing and vision, and calm nausea. Relief of conditions such as arthritis, sore muscles, pain, and osteoporosis; prevention of cancer, kills infection, conditions the skin, aids digestion, controls weight, preserves memory, slows aging of the body and mind and breakdown of joints, bones and cartilage. Small wonder the uses of vinegar as one of the natural ingredients extremely useful for your special pets safety and health.
It's possible he may have jumped down from a high roost or other high place and injured his leg...a sprained or pulled ligament, or tendon. You'll want to lower roosts and eliminate high places. Check the bottom of his foot for a round dark in color scab, it could be bumblefoot. If it's bumblefoot, minor surgery may be required. If it's a sprain, tendon or ligament problem...keep him seperated from the others. I recommend that you place him in a crate or cage to limit his movements. This will prevent him from causing further injury to his leg. Provide him food and water while he's caged. You can purchase vitamin B complex at a pharmacy, crush a few tablets into powder and sprinkle on top of his food to eat. The vitamin B complex may help speed up recovery. Do this for 5 days then release him, if he's still limping put him back in the cage for 7 days continuing the vitamin B complex in his food. Then release him again and see if there's improvement. If not, recage him and stop the vitamin B complex treatment. These types of injuries take time to heal; sometimes a week, a month, or never. Then it becomes a quality of life issue that you have to decide upon.
If he needs worming, I recommend Safeguard liquid goat wormer. Administer orally undiluted 1/2cc, repeat dosing him again in 10 days.
Also inspect him for lice/mites especially around the vent area.