My 26 Year Old Son Has Been Battling The Same

My 26 year old son has been battling the same come and go symptoms for several years now. He'll go a period of with having no symptoms to what seems to be a full blown attack of some sort.He'll get servers stomach pain, is unable to keep any food or water down for days, vomits non-stop etc. He's been to doctor after doctor but STILL no diagnosis or relief! We're lost as what to do, he's unable to work or live a normal lifestyle of any kind. PLEASE HELP!