Why would a 3.5 yr old frequently vomit even though he eats only a little?

My nephew is 3.5 yrs old, and he has this problem of vomitting food more than a couple of hours after food. What would be the questions that we should be asking the doctor? Is there any hereditary transfer of such a phenomenon, because my sister used to do the same thing until 5 years of age. Thanks in advance for the replies!