Why is my dog throwing up foam?

He may be throwing up because his stomach is empty or irritated. Please take your dog to the vet! Especially if this is abnormal for your dog.

From https://www.vetinfo.com/dog-vomiting-...

Dog vomiting can occur for many reasons, some mild and some quite serious. It will often be accompanied by other symptoms, which can help you identify the cause. The most common cause of vomiting white foam is some type of stomach upset. It will often occur if your dog drank water too quickly after exercising or ate something that disagreed with him, such as grass or an inedible substance. If the vomiting continues, visit your veterinarian to make sure your dog hasn't swallowed something that he can't digest.

Your dog also may be suffering from bilious vomiting syndrome, which usually occurs soon after your dog wakes up. This is caused by too much acid in the dog's stomach and is more common in dogs with digestive problems.

If it happens regularly, your dog may have an ulcer or inflammatory bowel disease and may need medical treatment...

White foam is one of the characteristic symptoms of bloat, which is the swelling of the stomach that causes it to rotate, trapping the stomach's contents in the stomach and obstructing veins around the stomach. This can quickly lead to death if not treated, so it should be addressed immediately.

Dogs will often cough up a yellow mucous or white foam when suffering from kennel cough, an upper respiratory disease contracted from other dogs. The most common symptom of kennel cough is a dry hacking cough. Additional symptoms include eye and nasal discharge, fever, lethargy and loss of appetite.

Similar illnesses, such as parainfluenza or other colds, may also cause your dog to vomit white foam. Usually, these will include other cold-like symptoms, such as eye and nasal discharge, sneezing and fever.

Vomiting white foam is a rabies symptom. Vomiting white foam can also be caused by illnesses of the digestive tract that need further treatment. Illnesses that can cause vomiting include hepatitis, pancreatitis, diabetes, and kidney disease.