Hello and welcome to Just Answer. I am a licensed veterinarian and would be happy to answer your questions.
Is he still vomiting?
Is he able to keep water down?
How is he acting? Is he lethargic?
Can you check his gums? Are they pink? moist?
Is the diarrhea liquid stools or just soft? Any blood or mucus?
Thanks and I will respond when you reply.
In most cases of a gastroenteritis with vomiting and diarrhea, simply fasting them will be enough to help them to start to feel better. In his case, if he has been vomiting through the day and into tonight, he is probably pretty miserable. If he is not taking in any fluids and still vomiting, we do have to worry about him getting dehydrated. It won't happen right away, but being a small dog, we don't want to go too long.
It is definitely a judgment call on your part. It is hard to tell if the lethargy is weakness or just because he is nauseous. If you think he is unstable or too uncomfortable, I would recommend that you have him evaluated tonight at an emergency clinic. They will be able to offer meds by injection to help his upset GI tract and probably some form of fluid support. If you are going to just monitor him for now, try offering a droppersful of water every hour or so. I he can't keep that down or starts to become more lethargic, I would take him in.
I hope that helps. Please let me know if you have any further questions.
It is always possible that a dog could have ingested something that may be toxic or serve as a gastric irritant. It is not easy to prove that and would not necessarily be treated any different than a gastroenteritis from any other cause. Vomiting can last for a day or two or for as long as the gastric lining stays irritated. If there was a possible poisoning, there may be other signs picked up on a physical exam, but in general, it would be hard to tell and the treatment would continue until the signs resolved.
If you are worried about that, better to error on the side of cautious and have him examined sooner versus later.