22 year old and constant vomiting?

my son is 22 years old. he has no serious medical problems and is in good shape. last april, he vomited for about 12 hours (we took him to the hospital and he was put on an IV). the next month, the same thing happened. again, we had to take him to the hospital because he was so dehydrated from vomiting. now, about 9 months later, he left to go on a cruise. he left on saturday, and for 72 hours, has not stopped vomiting. again, he was put on an IV because he was so dehydrated. he got home & everything was fine. just tonight, it started again. there is NO abdominal pain, and literally no side effects--it comes out of no where! please, please, please take some time & help. we are so desperate, the doctors don't know what to do. he is going in for some more tests next week but we don't think we can wait.

key facts:
-vomited for 12 hours straight (april, '09) location=college. no changes in diet
-vomited for 12 hours straight (may, '09) location=home, no changes in diet
-vomited for 72 hours straight (march, '10) location=cruise ship (he JUST got on the boat and immediately got sick)
-vomited for 5 hours SO FAR (today, 2 days after previous incident) location=home, ate salmon & pasta tonight..

thank you & god bless..