Has he had any scope for eating plants, or a lot of herbivore poo? (my dog used to adore sheep droppings, which caused green poo, though not diarrhoea) - if theres a likely cause for it, its less worrying isn't it.
I'd say if he seems OK in himself carry on as you say. Vastly overcooking the rice so it takes up loads of water is an easy way to ensure hydration. A bit of probiotic yogurt could be helpful too, maybe.