What is wrong with my son?

It started about 7 years ago.he suddenly got sick to his stomach,cant keep anything down lasted about 2 weeks he lost 30 pounds then suddenly got better, This seems to be recurrent, he gets sick 4-5 times a year each time lasting about 2 weeks. I have taken him to the hospital on several occasions, he has no insurance they give him fluids and a shot for nausia and send him home still sick. One hospital kept him for a week said they did all kinds of test but found nothing..