What Causes Random Vomiting At Least Once A Week?

Hello, My son has been throwin up at least once a week for the past two months. I have taken hime to see his physcian and he has been hospitolized twice due to dehydration. The doctors diagnosed him with Mono both times he was treated and relased but the throwing up never stopped. I am a bit fustrated and cunfused as to what is wrong with my child because i dont know where or who he could have gotten mono from. He has no fever no pain just the nausea and they are still sayin he has mono. Could u please help because i am runnin out of options