At 10-11 months old he is going through his first molt, which for Green Cheeks is the huge one where they completely change body colors (depending on their mutation). They molt their entire bodies, with the most molting on their head/face/chest/belly. He's also probably already started puberty or is going to very, very soon, usually by 1 year-old they are going through it, so it's a double-whammy for a lot of birds, as they end-up going through their first puberty and their first huge molt at the same time...
He's no-doubt very, very itchy and distracted by the numerous pin-feathers, so I would try a daily shower/bath for him; I don't know what Nico likes as far as baths, some Green Cheeks like mine will take a bath if you just give them any container of water, others prefer showers, etc. But a daily shower will soften-up the sheaths around the pin feathers and help calm down the itching, and will also help to shed the old feathers...As far as the puberty goes, make sure he's getting a full 12 hours of solid, restful sleep every single night, and more is even better, meaning you get him on a full "Natural Light Schedule", where is goes to bed with the sunset and he wakes-up with the sunrise, making sure that his cage or wherever he sleeps is in a quite room with a window so he can actually see the change in sunlight...This time of year it means that they go to bed pretty early, but it really does keep their hormones under control...
Also, remove any and all small, warm, dark places that he can get inside or under from his cage, such as any "Happy Huts" or "Snuggle Huts" (which are death-traps anyway and no bird should EVER have them), any tents, any type of boxes, any fabric/cloth/material he can get underneath or make a "nest" out of, such as towels, blankets, newspapers, any types of bedding, wood chips, paper towels, etc. And when he's outside of his cage make sure he cannot get underneath any furniture or get into any cabinets or any other small, dark places, as this can instantly cause hormonal rages that can be very aggressive towards you and very stressful to him.
Also, avoid all warm, moist, mushy foods like oatmeal, grits, any types of "formula", mashed potatoes, etc. These also trigger hormonal rages, as they mimic both the hand-feeding formula and the food fed to them by their parent birds.
It will pass, but it's a double-whammy and it's going to be stressful for both you and him...I would wait until you're certain his first molt is completely over and then watch him and make sure that he's not STILL over-preening or plucking/barbering himself...Right now you have no way to tell if he's actively doing any Feather-Destructive Behaviors due to the stress of being at a bird-sitter's and away from you for 2-weeks because it's perfectly normal for them to be constantly preening themselves and for feathers to be falling-out all over the place during this huge molt, but once it's over if he's still constantly preening or actively pulling feathers out, then you'll know he might be doing a little barbering/plucking...I'm assuming it's all due to his first molt, it's the nasty molt and it's really tough on them...
