First, welcome to the forum, and I'm so sorry your bird is plucking himself.
He should be taken to a vet to rule out an infection, first. If it isn't health related, there may be things you can do to at least hopefully slow him down.
This is a wonderful thread that discusses plucking, and it's a good place to start
What kind of bird? what is his/her diet? is he/she on seeds or pellets? does he/she have enrichment and can we see the cage setup? How often does he/she get let out of the cage? did he recently lose a mate? how old is he/she?
basically can we have the background of your sweet baby? I also advise going to see a vet for an evaluation.