When invited to dinner, it's usually customary to bring a bouquet of flowers for the host/hostess, a bottle of wine, or something for dessert. Because you mentioned he's health conscious and you're not sure if he drinks alcoholic beverages at all, perhaps a bottle of wine isn't the best choice. You can't go wrong with flowers, and if you wanted to bring an assortment of pastries or a small cake, a pie, etc., that would also be a good choice. Remembering that he's health-conscious, a small fruit platter you can buy in the supermarket or even make yourself, with a beautiful and healthy assortment of seasonal fruit, would also be a good idea for a refreshing dessert.
If he told you he's making rice and beans, it might be accompanied by chicken or he might be making 'arroz con pollo', which is a rice and chicken dish.
He sounds like a very nice man and I hope you have a wonderful time!
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