Welp, it finally happened--we got an infection in our crop. I fed him at 7 on Friday, his crop had barely emptied by midnight, and I sat up with him all night. Tried to feed him a little vinegar, but by about 2 AM his feeding response had vanished. His crop started expanding with bubbles and I got him to the vet when they opened at 7 on Saturday. He seems to be doing fine now, but they had to flush his crop (it looked all mucous-y, like snot) and give him some Baytril and subq fluids. I have the tools now to flush his crop again if need be, but fingers crossed he started emptying normally again once the sick contents were gotten out.
He's started to nibble on millet and fine pellets. He's also starting to get his adult voice. Still sounds like a baby most of the time, but now and again he'll start calling and sound like an actual adult cockatiel.
He's also figured out how to get out of his basket, so we've had to make a little "jail". I have a cage set up for him, but am keeping him in the basket for the next day or two for easier monitoring.

Had to get a crop bra because the infection caused bubbles that stretched his crop:

But he's doing surprisingly well!

He's much, much darker than Pippin:
