That is a horrible diet, just sunflower seeds, that could contribute to his problems, he needs a better diet, pellets, vegetables, a little fruit, grains, rices, legumes, a little meat like chicken now and then, some egg a couple times a week, a tiny sliver of cheddar cheese couple times a week and maybe a little yogurt now and then. Be sure to check the bird food room for ideas and suggestions and for the forbidden list of things they cannot have.
I have to say this vet does not sound very helpful, being an avian vet he should have some answers for you.