Hi welcome to health care magic........
Vomitting brown substance suggests hematemesis
i.e blood in the vomitus.
common cause is gastritis
or a AV malformation or a extrahepatic portal hypertension
. Kindly have a gastroenterologist opinion get a upper GI endoscopy
to rule out that all is well with the kid. Get a Heamoglobin tested to check his blood levels too .