Rachel, you really do need to get him checked out at the
vet. True, almost all plucking is either behaviorial or a
health issue. But until you rule out the health side of it,
you may very well be just spinning your wheels, and maybe
wasting his time, getting to the bottom of it. It might tie
in with the sitter but it may just as easily be coincidence.
Or, he could have been harboring something bacterial or
fungal for awhile and it took the stress of you being gone to
lower his immune system far enough for it to manifest. Don't
ever under estimate the toll stress can take on a parrot.
You are doing good things, if it's only a behavior problem.
But it's all wasted, if it's health based. Yes they do make
collars but for a stressed parrot, that would be my absolute,
last ditch effort. They hate them and it really jacks up the
stress level. Good luck with him.