Well since you're on your way to the vet - and I commend you for this, it's absolutely the right and most necessary thing to do - I'll try to help with the nutritional improvement you really need to start addressing despite his refusal to cooperate.
You have to look at it this way, would you allow your child to eat nothing but candy and ice cream for their entire childhood just because they have a tantrum every time?
Eventually, the child (in this case the bird) is going to show the health problems as the result of their diet.
--- Of course what you may be dealing with right now could be an infection ranging from bacterial to fungal or viral, but believe me, you're going to want to change the diet.
Even tumor activity and liver disease is higher in seed eaters.
A diet consisting mostly of pelleted food supplemented with fresh fruits, vegetables, grains and a good seed mix that does not include sunflower seeds, is your parrot’s best choice.
It’s not easy for some birds to make the conversion, but quite often that’s because their owner is too easily swayed by their demands and fear that the bird isn’t going to eat at all if they don’t get their junk food.
When I take in rescues, one of the most common problems is nutrition. The birds are either terribly underweight or obese. Changing their diet is often a matter of life or death and I haven’t lost anyone to date.
I’ll try every type of pellet out there, whether fruity or plain, spicy or a combination – just be sure to get the size & type appropriate for your individual bird.
Offer a pellet from your fingers as a treat (if your bird is used to taking treats from your fingers that is) and go ahead and try one yourself so the bird can see. I’m serious – try it yourself. Your bird shouldn’t be expected to eat anything that you wouldn’t eat yourself.
I’ve mixed pellets in with cereal too, especially a good, healthy, low sugar type. Try crushing them into an all natural yogurt or baby food of mixed vegetables, sweet potatoes, squash or the like. One of our macaws started to love them when she found them in with her blueberries and other cut up fruit.
I’ve found it’s not a good idea to mix the pellets in with the seeds, but be creative otherwise.
One warning is that if you mix the pellets in with anything wet or even make a ‘mush’ out of the pellets using plain water, a natural, low sugar fruit juice – you must remove the dish (must!) after an hour or two, tops. There’s too much chance for bacterial growth in wet foods and this only makes a problem worse.
Sprouting fresh seed for vitamin and enzyme loaded greens for your bird is easy. The rumors about it being dangerous, often referring to fungal contamination or bacteria, are greatly exaggerated and not a concern when sprouting is done right.
Soaking seeds in an even cleaner environment can be effected by adding 1 tablespoon of GSE (Grapefruit Seed Extract) to a gallon of plain water and then using that water to soak the seeds.
When rinsing the sprouts, use a fresh batch of treated water.
One of the very best and most comprehensive guides I’ve seen on sprouting, including detailed instructions and explanations of the different kinds of seeds and nuts is here
For overall feeding and nutrition for your bird, this is a pretty extensive resource that includes homemade recipes that are for both birds and their owners to share
Good luck and please let me know how you make out.