We are south west Western Australia - on acreage and the chickens have a lot of access to green grass, this particular one is always eating it and has always had really watery diarrhoea. There is another rooster in with him who may be keeping him away from the food so that may well be compounding the situationWhat is your location on the planet Earth? We need to know where you live so we can know the conditions that your rooster lives under.
Has he always shunned feed and preferred grass? If so, that may indicate he has a condition called "pica". It's an eating disorder where strange things are selected to eat, a obsession. To help the rooster, he must be kept away from the grass for now.
He probably has a serious vitamin deficiency. He needs vitamin E and the B-complex @SmiYa0126 mentioned, and a good poultry multivitamin solution.

