The rat/mouse feces shouldn't be too much of a problem - anything though can cause digestive upset. This can also be caused by stress. If he is acting fairly well otherwise it is probably okay to treat at home until Monday. If he begins to be very lethargic, seems painful, or is unable to keep anything down I would get into the vet sooner. The normal recommendation for treating digestive upset at home is to fast the dog for 12 - 24 hours. This may not be much of a problem since he doesn't want to eat. Many times though people keep offering special and new food to encourage them to eat but just make things worse. After the fasting period you feed a bland and bulky diet of boiled chicken and rice (2/3 rice) for two or three days. Then slowly transition back to normal diet over another couple of days. There are also several over the counter medications that can be used in dogs. Pepto is one of them - I have found Pepcid to be helpful as well. My dog was prescribed 5 mg twice daily when she was about 8 pounds. Here is a link to more information about these medications.
If he is refusing to eat anything after the fast and even during the fast since he is so small - you may want to give him a little karosyrup to prevent hypoglycemia. Just rubbing some on his gums from time to time can help.
I hope this was helpful. If you need more information - just reply to your question.
____________If this was helpful, please click ACCEPT_____________