Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Alfie is vomiting today, the contents of which now look green and lumpy.
Dogs that are very nauseous will eat grass, but eating grass when he is already vomiting will only further irritate his stomach and make him vomit more. You need to stop him from eating any more grass.
Dogs that have a gastrointestinal blockage or very abnormal intestinal motility often vomit green material as that is digested bile, bile that sits in the stomach for a while. Bile only refluxes back into the stomach when gastrointestinal motility is abnormal, such as with a blockage or poor gastrointestinal motility due to primary intestinal or metabolic disease.
Vomiting can be related to chronic pancreatitis, a dietary allergy or sensitivity, toxin exposure, a viral or bacterial infection, inflammatory bowel disease, internal organ failure, a full or partial gastrointestinal obstruction or even infiltrative cancers such as lymphoma.
Dark/black liquid or tarry stools usually indicate digested blood in the stool.
Is he a pup that gets outdoors unsupervised such that he could get into something you don't know about?
Or is he a pup that eats things like bones, toys other things he should not?
Ideally he would see a veterinarian since he has some very concerning symptoms. They could examine him, run blood tests and possibly check radiographs and/or an ultrasound to evaluate him and know best how to treat him.
In the meantime they can administer injectable anti-nausea drugs, coating agents to ease gastrointestinal irritation, and fluids to rehydrate him.
If you cannot have him seen for whatever reason there are some things you can try at home, though if he doesn't respond quickly he should see a veterinarian.
At home to try and settle his stomach you can give either:
1) Pepcid ac (famotidine) at a dose of ¼ of a 10mg tablet per 5 to 10 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of ¼ of a 20mg tablet per 10 to 20 pounds of body weight every 24 hours
These are acid reducers and may help him feel better. They are quite safe and can be used for several days if necessary.
A couple of hours after giving the acid reducer I recommend offering a small amount of water of clear broth.
If that stays down for a couple hours tomorrow morning offer a bland diet. A bland diet is a mix of 2/3 boiled, minced, white skinless chicken and 1/3 boiled, plain, white rice mixed with some low salt chicken broth to make it easy to lap up and swallow.
If Alfie continues to not eat and continues to vomit green material he should see his veterinarian or an emergency veterinarian for an examination, diagnostics and intravenous fluids and supportive care.
Please let me know if you have any further questions.