Don't wait until Monday. Get your dog to the emergency vet ASAP.
I hope you were able to take your dog to a vet today. His symptoms sounds very similar to my dog's when she was in kidney failure. Unfortunately, there isn't anything to treat that condition. Once those kidney cells are done, they're done, and they don't regenerate. Calcium supplements like Tums tablets can help maintain what function remains, but won't stop the failure from progessing.
Since your dog isn't eating, that's why he isn't pooping. It's water intake that is most important. If he isn't drinking, then dehydration can become extremely dangerous. If he is't drinking and his gums look very pale, he needs to go to the ER vet.
I hope it's NOT kidney failure or anything more serious, but if he is dehydrated you'll need to make sure he sees a vet before Monday. If you don't already know of an ER vet in your area, now is the time to find one. Call your own vet clinic as they usually will leave an outgoing message telling you who to call and where to go for an emergency.