It sounds like your rooster is very ill. I have one doing the same thing, not being able to jump up to a low roost for the last 2 weeks. Mine is over 7 years old. How old is yours? First I would offer him a few drops of Poultry NutriDrench, up to 3 ml daily. Also offer water up to his beak, and some watery chicken feed with a little egg added. Look him over for mites and lice, and if found, treat him with Permethrin 10 or the poultry dust. Check his legs for scaly leg mites as well. Feel of his breast bone to see if he has lost weight. Check his crop to see if his crop is empty, full, firm or puffy. Look for diarrhea and check his vent. Listen to his breathing, and look for any eye or nasal drainage. Sorry about your rooster, and I hope that you can help him.