It could be as simple as overeating. Does he have to compete with another dog for food and therefore may be eating too much too fast? I would also worry that he has a piece of rawhide in his stomach that he is trying to get rid of.
I would begin by taking a rectal temperature - normal is 101 to 102.5.
Here's an article you can read on the different causes of vomiting.
http://www.vetmed.wsu.edu/ClientED/vomiting.asp