My son is vomiting uncontrollably and is vomiting what looks like brown coffee grounds what causes that?

Blood in stomach. Blood mixes with acid in the stomach and looks like coffee-grounds. The blood could be from an ulcer in the stomach, or could come from a broken blood vessel or capillary in the esophagus.
Go to a doctor now. If you are vomiting coffee ground colored material, you should be seen by a doctor. Please make an appointment as soon as possible. It is very important to determine if there is bleeding in the stomach that could be causing the symptoms.