Hello, Thank you for your question. My name is Rebecca. I am a veterinarian and will do my best to help. There are many reasons for a dog to vomit: infections, parastes, liver disease, kidney disease, inflammatory bowel disease, cancer, pancreatitis
, something they ate that is upsetting to the system, too much stomach
acid, eating grass, hormonal imbalances. The green color does not narrow it down much; that could be from grass, bile, partially digested blood, eating something green. If there is any weight loss, excessive thirst, or reduced appetite, I would more concerned this could be serious. If his gums are pink, he is eating normally, not losing weight, and not drinking more than usual, it may not be serious. If it seems to be happening more frequently I would be worried. The first step to finding out why he is doing this would be a thorough physical exam: listen to heart and lungs, palpate internal organs and lymph nodes, take his temperature, feel his pulses, etc. You could try giving him famotidine
(Pepcid AC) once to twice daily. The dose is 5 mg per 10 to 20 pounds. I hope this was helpful. If you have any more questions please let me know.