Reoccurring sever stomach pain every month

[Lisam386]

My 21 yo soon keeps getting this reoccurring severe stomach pain for over a yr. Along with nausea and vomiting. This happens every month like clock work. He can't eat, sleep, drink, and is in so much pain. Nothing seems to help.i take him to the ER and the docs dismiss it as a stomach virus..He can't hold a job because he keeps missing time due to this stomach thing. I don't know what to do..please someone point me in the right direction