Hello & welcome, I am Dr. B, a licensed veterinarian & I would like to help you with your wee one today.
Now this is a very challenging situation you have here. Unfortunately, we are a bit limited in what can be done if there is no veterinary services in reach (even with a trek). You vet did initiate the symptomatic care we do use with these situations but if this pup is listless and vomiting; we'd have wanted him to stay on IV fluids and be treated with injectable anti-nausea treatment. Since that is the ideal and pups are so fragile, I do just want to note that you can check these databases for your nearest emergency options just in case:
http://www.vetlocator.com/ and www.veccs.org/index.php?option=com_hospitals&nationid=1&Itemid=193
Now since you noted he was listless and pups have small body reserves we do need to be proactive here. This lethargy is suggestive of a possible blood sugar crash (since small breed pups are at real risk of this when vomiting ). Therefore, if he does seem weak, wobbly, or dull; you can rub karo syrup (or honey, pancake syrup, etc) on his gums to give him a sugar boost.
Otherwise, we can try some symptomatic care to settle his stomach. To start, you can try this pup with an antacid to soothe his stomach. Common OTC options are Pepcid (More Info/Dose @ http://www.petplace.com/article/drug-library/library/over-the-counter/famotidine-pepcid) ,or Tagamet (More Info/Dose Here @ http://www.petplace.com/article/drug-library/library/over-the-counter/cimetidine-hcl-tagamet). These are usually given 20 minutes before offering food (to allow absorption) and of course you want to double check with your vet if your wee one has a pre-existing condition or is on any medications you haven't mentioned. Though if he cannot even keep that down, then that will be a red flag that we need him seen.
Once that has had time to absorb and is more steady on his stomach, you can consider starting him on a light/easily digestible diet. If you do so, start with a small volume (a spoonful) to start. Examples would be cooked white rice with boiled chicken, boiled white fish, or scrambled eggs (made with water and not milk). There are also veterinary prescription diets that can be used in cases of gastroenteritis (ie Hill’s I/D or Royal Canin’s sensitivity). When you offer that spoonful, give him 30 minutes to settle. If he keeps the food down, you can give a bit more and so on. As his tummy stabilizes, you can offer more. The aim of the easily digestible diet is that it will be better tolerated and absorbed by the compromised gut. Therefore, it should get more nutrients in and result in less GI upset. As long as improvement is being seen, I usually advise that the diet be continued until the diarrhea is settled, and that they are then slowly weaned back to normal food over a week.
Since vomiting can quickly dehydrate a pup, we need to keep an eye on his hydration. The reason is because no matter how much they drink, we often find that dogs just cannot keep up with diarrhea fluid losses for long (and dehydration is what makes them feel poorly). To check his hydration status to make sure they are not becoming dehydrated there are a few things we can test. Further to gum moisture, we need to make sure his eyes aren't sunken. nor that he has a "skin tent" when you lift the skin. To see how to check these parameters for dehydration, you can find a wee video on this HERE (http://www.ehow.com/video_12232503_dog-dehydrated.html). If you are seeing any signs of dehydration already, since he is young, then you do want to have him seen by the vet before this becomes an additional issue.
If you are concerned that he is becoming dehydrated, you can try and encourage him to drink but offering fresh water or even low-sodium chicken broth. Don't be tempted to syringe feed fluids at this stage, since this is contraindicated for vomiting dogs (since it can make them vomit more).
Overall, this situation is far from ideal and it is a shame his vet cannot aid you more over this weekend. In this case, you can try the above just now to try to stop his vomiting, keep him eating, and stable over the weekend. But again if he doesn't pick up, then it may be best to make a long trek to the nearest ER to just be safe and get him back onto fluids and anti-vomiting medication by injection.
Please take care,
If you have any other questions, please ask me – I’ll be happy to respond. Please remember to rate my service once you have all the information you need as this is the only way I am credited for assisting you today. Thank you! : )