I have a very dear friend who has what appears to be an almost identical situation. He was smoking large quantities of weed (1/4-1/2 oz a week) for over a year and supposedly a traumatic emotional situation with his room-mate and heroin addiction caused it. When I lived with him, he was chronically smoking weed and would get into these vomiting fits every few weeks where he could not keep down any liquid and would live by the toilet or in the shower until he was so dehydrated he would admit himself to the hospital and go on a saline drip and he would ALWAYS get better. This happened over 4 times in a 10 month period. After a particularly bad episode, he had surgery where they found out his intestines had folded and thought that was the cause. They unfolded it and he was episode free for over 6 months. During the ENTIRE year I was living with him, he thought the cannabis was actually helping him and when a surgery seemed to solve the problem, I think it validated cannabis was not at the source of the problem.
Fast forward to this year, he is having episodes again and has been in the hospital three times already. He has seen SO MANY different types of doctors but I am highly suspicious that the root cause of all his medical problems is chronic cannabis smoking. He got down to an 1/8th or less during my time with him a week but it seems that it is very black and white and he has developed an allergy to cannabinoids. you can message me for any more questions or concerns. I would be happy to lend any advice or help to you or your son.
Much love.