I think if I were you I'd cross post to the emergency/help needed unless @Mixed flock enthusiast or @R2elk responds. Guineas resist many illnesses but can succumb quickly if they do get sick.
Sorry to hear about your sick bird! Guineas can get some of the same diseases that chickens can, like Mycoplasma, which can cause respiratory disease. There are a lot of other problems though that can cause the kinds of general “sick” signs that you describe. @Sydney65 had a good suggestion about posting to the emergencies and diseases forum. I hope they can help your Guinea! In the meantime, it sounds like you are doing the right things: keeping him separated, warm, offering food and water, electrolytes wouldn’t hurt, and just general nursing care. If a vet is an option, you might consider that. You can also look into your state diagnostic lab to help you figure out what the problem was if the worst happens. That’s especially important if you have a flock disease. Best of luck - I hope you’ll let us know how he does!
