What Can I Do For My Guinea Pig With Diarrhea?

Call the vet immediately. This is the best thing you can do for your guinea pig. This might be something very serious. Avoid giving him good treats and snacks, just feed him pellets. Don't take it lightly, take your guinea pig to the doctor as soon as you can. The following website will be helpful for you to take care of your guinea pig in the future.
 
www.cavyspirit.com

Also it might have wet tell, vet will give you medicine that you put in it's water,

What shall I do if my guinea pig has blood and mushy poo coming out of its bum at the same time