Did he get into any of the following:
wet or soured feed
mildew, algea, stagnant water, a pond
compost pile
maggots
canned food thrown out or other kitchen debris
decaying vegetation, including hay or yard clippings
droppings collecting under pens
a manure pile
Anything at all that might have gone bad - fungus?
If so, I would highly suspect botulism. One thing you will want to do is pull at some feathers; if they easily fall out, it IS botulism. If so, you will want to use one of the flushes recommended at the MSUCares SOlutions for Poultry site:
http://msucares.com/poultry/diseases/solutions.html
As for the present, because this is so sudden, I suspect toxins. Search your brain (and your place) for anything he could have gotten into.
In the mean time, it's very very important that he's hydrated correctly. I would give him a poultry electrolyte solution or vitamins/electrolytes immediately. He won't drink, might not be able to. You will have to offer some at the side of his beak to dribble in. Do not pour in his throat.
If you don't have a poultry electrolyte solution, you could use Pedialyte or gatorade in a pinch. I would also consider mixing a boiled egg's yolk in some of the electrolyte solution and making a thick soup of it. Offer that as well as the water dribbles. A little plain yogurt in that soup may also be useful - as that sort of diarrhea really thins out the necessary good gut bacteria.
Does the diarrhea appear very green and smell quite bad? Or is it a different color?
To be honest, and I try to stay positive, but this is a very serious situation. A bird this extreme and likely dehydrated will have a hard time healing - but it can happen. You will have to do the work of nourishing him and hydrating him.
Also, if you have dog pee-pads, diapers, etc - that will help with the diarrhea cleaning.