Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I'm sorry to hear that Charlie has diarrhea the last few times he passed stool and vomited today.
In most cases diarrhea and vomiting are triggered by eating something they should not, too much table food, too many treats or something they find outdoors. More serious causes include viral or bacterial infections, chronic pancreatitis, a dietary allergy or sensitivity, inflammatory bowel disease, internal organ failure (kidney or liver disease), Addison's disease (a poorly functioning adrenal gland that cannot respond to stressful situations) a full or partial gastrointestinal obstruction or even infiltrative cancers such as lymphoma.
Because he is a younger pup internal organ failure and cancers are less likely but I mention them to be complete.
In a young dog, especially if he hasn't finished his vaccine series, a viral infection such as Parvo virus or a foreign body leading to a partial or full gastrointestinal obstruction would be the most likely causes.
If he seems to be getting sicker then I would also be very concerned that this might be related to Addison's disease given that he seems to be the only pup affected in your home.
Worms can cause loose stools, but rarely cause vomiting.
If he continues vomiting and this has been going on for more than a day and if he is drinking poorly too ideally he would see a veterinarian for hands on care. This can easily lead to dehydration and he needs fluids if he continues vomiting, as he won't be able to drink, keep it down, and hydrate himself.
There are some things we can try at home, but we cannot replace in clinic intravenous fluids and injectable medications so if he isn't responding quickly.
To try and settle his stomach and get him to stop vomiting you can give either:
1) Pepcid ac (famotidine) at a dose of one half of a 10mg tablet per 10 to 20 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of one half of a 20mg tablet per 20 to 40 pounds of body weight every 24 hours
These are acid reducers and should help him feel better and hopefully get him to eat. They can be given for several days if necessary as they are quite safe.
I would pick up all food for now and water for a couple hours to allow his stomach to settle after the acid reducers.
In a couple hours when you give him water make sure it is in small amounts only. To get some electrolytes in you can also offer him a 50:50 mix of pedialyte and water.
Today even with the fast you can start Kao-pectate at 1/2ml per pound or 1/2 tablespoon per 15 pounds of body weight every 8 hours. This is quite safe and will coat his irritated gastrointestinal tract as well as absorbing bacterial toxins. You can find kao-pectate at the drug store.
Tomorrow morning offer a bland diet of 1/3 boiled minced white skinless chicken or lean hamburger and 2/3 boiled white rice mixed with some low salt chicken or beef broth to make it easy to lap up and swallow. If he has blood or mucous in his stools or is straining repeatedly to pass stools then you can add 1-2 tablespoons of canned pumpkin (not pie filling, just pumpkin) to each meal. Pumpkin is high in fiber and will help soothe an irritated colon.
If he refuses the bland diet you can offer a little meat baby food. If he refuses both then don't push it, he needs hands on veterinary care at that point. But if things go well and he does eat and doesn't vomit and his stools firm up feed him the bland diet for 2 to 3 days then slowly start to mix back in his regular food, a little more at each meal. It should take about 5 to 7 days to slowly convert him back to his regular diet.
If he continues to vomit and have loose stools, runs a fever (more than 103F rectally), has a tense painful belly or if he refuses to eat he should see his veterinarian for an examination, diagnostics, injectable anti-nausea drugs, intravenous fluids and supportive care.
Well worth checking at least a couple fresh stool samples too even if he seems to improve as parasites commonly cause loose stools. Their eggs/cysts are shed intermittently and they may be picked up on the second sample or third test even if the first sample tests negative.
Please let me know if you have any further questions.