Hello, My Son Is 36 Years Old. He Has Been

Hello, my son is 36 years old. He has been very healthy all his life and does not drink, smoke or do drugs. He has developed a condition that he starts with feeling bloated, vomits, shakes violently and sometimes runs a fever. It was happening about once a year but is happening more often. he went to the ER once with it. they did an ultrasound and said they didn't see anything that would be causing it. Said to come back whne it happens again