Just checked the reference "The Large Macaws", your bird is on target as far as weight. He should be getting 60-100cc of formula 2-3 times a day, about 200cc of formula for a complete 24 hour period whether 2 feeding or 3. Green formed poop is great but I have to agree that he may be eating something other than formula, the white you see means he is digesting the food.
I would find some Bragg brand apple cider vinegar and add a couple of drops to his formula as a precaution as well as it being good for the bird. Should you go back to the vet have him do a fecal and gram stain and a wet mount of crop contents, he will be able to see if there is a yeast or bacterial problem.
Keep a weight chart of your birds weight and how much you are feeding.
Picture of the bird poop might help.
gary