I'll also chime-in here and tell you that #1) His head looks like he lost a bunch of feathers at one time, and these are simply a bunch of pin-feathers, or immature feathers, growing back in, and is completely normal...and then #2) His diet is absolutely horrible and is going to result in him developing Fatty Liver Disease, if he hasn't already, it's likely he already is suffering from it, and will eventually end his life young.
A "steady diet of sunflower seeds, nuts, and fruit" is made-up of exactly the 3 worst things you could feed him as his regular diet, and all 3 of these should only be given to your bird as occasional treats...Both Sunflower Seeds and Nuts contain massive amounts of fats, and this fat in captive birds just accumulates in his liver, causing liver disease quickly...And fresh fruit is unfortunately full of sugar, which is converted to fat and also just stored in his liver...If you don't change his diet soon, he's going to become seriously ill and his lifespan will be severely shortened.
The good news is that Fatty Liver Disease/Liver Disease in-general is reversible if you change his regular, daily diet to one that is low in fat, low in sugar, and high in healthy protein, vitamins, minerals, amino acids, whole-grains, healthy legumes, etc.
His regular, daily diet needs to consist of a "staple", or main-component, of either a Natural-flavored Pellet, preferably that's what he will eventually transition to having be his main diet, or a healthy, low-fat, low-sugar Parrot Seed-mix that contains NO Sunflower Seeds, NO Peanuts or other Nuts, and No Corn Kernels or Cracked Corn, but that contains a mix of healthy, low-fat seeds, grains, legumes, etc. I would start by ditching the Sunflower Seeds and the Nuts completely, and just giving him a healthy, low-fat Seed-Mix with NO Sunflower Seeds, No Nuts of any kind, and No Corn, and then slowly start transitioning him over to a Natural-flavored Pellet as the main staple of his daily diet, with only a very small amount of the healthy seed-mix each day...
In addition, he should be getting fresh Veggies every single day; any type of Veggies and Dark, Leafy Greens are great for him EXCEPT for Onions or Leeks, which are toxic to birds, and any of the "Iceberg" or watery Lettuces. He should be getting fresh Veggies every single day along with his staple of Natural Pellets and a healthy, low-fat Seed-Mix...
Fresh Fruit should only be given 3-4 times a week as a treat, in small quantities, as it contains too much sugar to be given every day. And then please only give him the occasional Sunflower Seed or Nut as treats, and only a few a day.
***The other thing you need to realize is that not only is a "steady daily diet of Sunflower Seeds, Nuts, and Fruit" causing him Liver Disease, but it is also causing Nutritional Deficiencies for sure, as it is not providing him with the vitamins, minerals, amino acids, healthy proteins, etc. that he needs to be eating every single day...It can also cause Diabetes, as well as eventually causing him to become Overweight/Obese, and this commonly causes the formation of multiple Lipomas, or Fatty Tumors, all over their bodies...