Brian is going through a molt. When new feathers grow in, they have a sheath around them that is removed during preening. In the wild, mates/other flock members take care of the feathers the bird can't reach, such as head ones. But in captivity, solitary birds don't have that option.
Helping parrots preen their pin feathers can be a great bonding experience. I frequently do this with parrots at the rescue; however my greys and caiques are experts at getting their own head feathers -- they don't need any help. Rocky does need help, but obviously that task falls to Thomas as I would only upset him and get bit should I try!
Brian is another story. He obviously needs help, but tries to bite us when we offer aid. You can see how he has a ton of pin feathers on his head: