Great advice above...You need to take him to see a Certified Avian Vet or an Avian Specialist Vet, especially since he is not eating...And please do not use any products that you can buy at a pet shop or over the counter, as you have no idea why he is pulling those feathers out. It could be something like a certain species of mites, it could be a feather-destructive behavior due to an environmental cause, it could be the other bird he is housed with pulling them out (unless you've actually seen him pulling them out himself obviously). So without knowing what they cause is, you cannot treat it at home, and certainly not with anything from a pet shop. Birds are extremely sensitive to everything and anything, and you could very-likely end-up making him worse by using something on it, or you could cause a whole other issue with that stuff.
If he does in-fact have something like mites, then it's also likely that your other bird has them too if they are housed together or kept in the same room. Until you get him to an Avian Vet, which needs to be done ASAP since it's effecting his appetite and could be a very serious issue, you need to separate him from the other bird into his own cage, and put his cage into a separate room from the other bird, as you need to do everything you can to protect the other bird from contracting the same issue if they haven't already. Also, you need to totally disinfect the cage that he's been in, and everything inside of it, the toys, perches, bowls, etc., in very hot water and bleach or F10 disinfectant, and then rinse it very well, as if it is some kind of parasite, they will be all over the cage and everything in it.