Hi there - I'm sorry that Milo isn't feeling well today. I see that he is a yorkie mix - small dogs can sometimes become hypoglycemic (that is, get low blood sugar) when they don't eat, vomit, or have diarrhea. There are a lot of reasons that a young pup could start vomiting from simple things like eating something out of the ordinary all the way up through life threatening illnesses like parvovirus.
Unfortunately it's difficult for us to know exactly what has triggered his vomiting, especially without examining him in person. The best course of action is to have him examined in person by a veterinarian, especially if he seems lethargic, sleepy, or is unable to keep any food or water down.
If you're absolutely unable to have him examined, you can try offering him ice cubes to lick on rather than larger portions of water. Rub some karo syrup or corn syrup on his gums every hour or two - this may help fend off hypoglycemia. If he's able to keep his ice chips down through the night, offer him a small amount of water and a tidbit of food in the morning.
Hypoglycemia can cause severe lethargy, muscle twitching and even seizures or death, especially in very small dogs, so do watch him closely for these signs. Depending on the cause of his vomiting, he may or may not get better on his own, so do keep a very close eye on him and let me know if I can be of further assistance.