Hey, I'm so sorry your Boo isn't feeling well. In my opinion if he really is acting not like himself then I would take him in and have a vet go ahead and do an exam. Metronidazole I wouldnt' give right now unless you get it okayed through your vet. He's presenting signs of vomiting and diarrhea not just diarrhea. The yellowish color is mostly just bile produced. I don't necessarily think his stool should be tested at this time.Usually that is a protocol in most clinics though when you take him in just to make sure there isn't anything that could be causing the diarrhea per se such as an overgrowth of bacteria ,worms,protozoan parasites.The main thing you really need to be careful of ,because he is so little with the vomiting and diarrhea is becoming dehydrated. This can lead to other issues. You can try and hold off the food for the day. Let his stomach settle and give him a bland diet of White meat chicken boiled with no skin and bones and add some cooked white rice no salt added and mix it together. Give him a little bit of that and see if he eats. Usually after holding off on food and giving them a bland diet the next day he will eat it. You can also try your vet and see if you can pick up some I/D canned food. It is usually prescription but if you have been seen for the diarrhea you may have already tried some and they should let you get it. If all else fails and you think you really need to take him in then please do so. I feel for the little dogs that present with these symptoms. Nonetheless It could be something other than just an upset stomach.