I suggest that you withhold food for the next 4-6 hours. When the vomiting stops and he can hold down water, you can feed him very small amounts of boiled chicken (no bones, no skin) and white rice. If this stays down, continue with it. Don't feed dog food, table scraps or treats for the next few days.
When the vet reopens, you will want to see the vet for a fecal exam to rule out parasites, the most common cause of what you are seeing.
If he seems distressed between now and then, you should find an after hours vet to see him.
Good luck!