In the wild, the parents actually will feed the chicks for up to a year.
You are going to have to show Rocky what foods are good to eat. Cockatoos are flock eaters. If they see a flock member (you) eating something, then they know its safe to eat.
Make up a plate of pellets, veggies, fruit, and some people food. Sit on the floor or at the dinner table and have Rocky there. YOU start eating the food. Make a big deal about how good it is. Make all kinds of yummy sounds, BUT don't offer any to Rocky. You almost want to hog it for yourself. Rocky will try to steal some off the plate. When that happens you are on your way to weaning. Don't worry if Rocky just holds it or takes a nibble just keep at it. Have Rocky eat with you breakfast, lunch, dinner, and snack time. Rocky will come around.
Cut the food into different shapes: stick, rounds, chunks, and such. Also, raw, slightly cooked, and fully cooked. *MAKE SURE ALL COOKED FOOD HAS COOLED.* The raw I find is better accepted if fed warm or room temperature.
Also, as Janny stated, track Rocky's weight. You can pick up a ounce/gram scale for about $25 at the office supply store.