What do I feed my rooster that has cold like symptoms ?

Commercial chicken feed is actually not the same as scratch, it is formulated with the proper levels of vitamins and minerals and is far more nutritious and balanced then plain scratch. For that reason it is a good idea to encourage any bird to eat as much of it's regular chicken feed as it wants. In addition to that maybe some chopped, hard boiled or scrambled egg, meal worms are a nice protein boost. It is best to give food that is easy to digest/move through the crop since sick birds crops may not function optimally.

As far as his illness, keep a close eye on him. If he develops wheezing, coughing or rattling breathing or gets very congested he would most likely benefit from antibiotic's. Chickens don't get colds, they get respiratory diseases and they very easily morph into pneumonia and secondary bacterial infections. The antibiotic's won't cure whatever he has and he will remain a carrier after he recovers, but it will help prevent death from those complications.