Raise his food and water higher on a piece of wood or block if you can, this way he doesn't have to bend over to reach them.
He's willingly eating/drinking when you bring him out of the crate which is good, so if possible, let him out of the crate several times a day. Some birds just seem to shut down when crated.
The white discharge is concerning. It may well be Vent Gleet. Does it have a distinct yeasty odor?
Is his crop emptying overnight? Take out his food/water tonight when it's time for him to go to sleep. Check his crop first thing in the morning before he's given anything to eat/drink. The crop should be flat/empty. If it's not then let us know.
A lot of white in the poop can be from dehydration, so make sure he's drinking well.
The links that
@Hanathehappyhen gave you about Vent Gleet are good treatment guides.
Give him a wash up and see if the discharge returns, if it does, then you'll want to begin treating him.