I don't think anyone really knows for sure why a bird plucks, and one size does not fit all on this one as well.
Blood tests will rule out any physiological causes.
Psychological causes? Anyone's guess.
Some folks swear by plucks no more. Others consider it snake oil.
Some folks swear by increased foraging, and added preening toys (especially home made toys from molted feathers.)
Increased bathing, aloe vera gel on the skin in the areas he tends to pluck, the use of sweaters, more structured interaction, redirecting the bird if you catch him plucking... ETC.
PERSONALLY, MY TAKE ON PLUCKING IS IT'S THE HARDEST THING IN THE WORLD TO DEAL WITH, AND TRY ANY AND ALL OF THE ABOVE. HOPE SOMETHING WORKS.