Frying a rooster rather than roasting?
When meat and poultry are tough it is because because of connective tissue which transfers the muscle's work to bone. The harder a muscle in an animal works the more connective tissue it will have and the tougher (but generally more flavorful) the result will be. Connective tissue (collagen mostly in muscles) breaks down slowly in the presence of moisture and heat. 

Frying is too short a cooking process to break down collagens, you need a low and slow method. If it were me I'd braise it instead of roasting it, so I'd coat it in flour and fry it off in a pan, then bake low (maybe 225f or 100c) it in a covered pan with a 500ml (about a pint) of chicken stock and a bit of rubbed sage for about 3 hours. If that doesn't make a tender rooster nothing on earth will.