For the other dining spots, he could wear shorts or slacks, depending on what he's most comfortable in, with a casual shirt. I feel that T-shirts are for the beach, not the restaurants, but alot of guys will wear them in the evening. Just so you know, the temps in the Piano Bar - a MUST-DO after dinner spot - are very cool, we always bring a sweater or light jacket!
I would always put on a nicer collared shirt, dress shorts and leather sandals for all of the restaurants except Otaheite, where I would have a pair of dress pants and shoes. I think you'll find a wide variety of dress, but I always felt "middle of the road" comfortable dressing like this.